The member of the congregation has to follow his imam with regard to matters that are subject to ijtihaad

Firstly: 

It is narrated that the Prophet (peace and blessings of Allaah be upon him) forbade making Witr prayer resemble Maghrib. See question no. 38230. 

The scholars differed concerning the case where the imam does something in the prayer that the person praying behind him thinks is not allowed, or if the imam omits to do something that the person praying behind him thinks is obligatory – should he follow him or not? 

Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) mentioned the scholarly dispute and favoured the view that he should follow the imam so long as the matter is something that is subject to ijtihaad. 

He said in al-Fataawa al-Kubra (2/117): 

They disputed regarding a case where the imam omits something that the person praying behind him thinks is obligatory, such as if he omits to recite the Basmalah, and the person praying behind him thinks that it is obligatory, or he touches his penis and does not do wudoo’, and the person praying behind him thinks that wudoo’ is obligatory in that case, or he prays wearing tanned leather from an animal that died naturally, and the person praying behind him thinks that tanning does not render the animal skin pure, or he is treated with cupping and does not do wudoo’ after that, and the person praying behind him thinks that one should do wudoo’ after cupping – the definitive correct view is that the prayer of the person who prays behind an imam is valid, even if the imam makes a mistake, because it is narrated in al-Saheeh that the Prophet (peace and blessings of Allaah be upon him) said: “They lead you in prayer and if they get it right, it is for you and for them, and if they make a mistake it is for you and against them.” Similarly if the person praying in congregation follows someone who recites qunoot in Fajr or Witr, he should recite qunoot with him, whether before or after bowing, and if he does not usually recite qunoot, then he should not recite qunoot with him. If the imam thinks that something is mustahabb but the people praying behind him do not think it is mustahabb, and he omits it for the sake of harmony, he has done well. An example of that is Witr: there are three scholarly opinions concerning it: 

1 – That it can only be offered as three rak’ahs joined together, like Maghrib, as was the view of some of the people of Iraq;

2 – That it can only be a single rak’ah offered separately from those that come before it, as was the view of some of the people of the Hijaaz;

3 – That both are permissible, as appears to be the view of al-Shaafa’i, Ahmad and others, and this is the correct view.  

If these people are of the view that it should be separate from whatever comes before, and if the imam is of the view that it should be separate, but the people praying behind him favour the view that Witr should be like Maghrib, and he goes along with them in the interests of harmony, then he has done well, as the Prophet (peace and blessings of Allaah be upon him) said to ‘Aa’ishah: “Were it not that your people are new in Islam, I would have knocked down the Ka’bah, made its door level with the ground, and I would have given it two doors, one through which the people could enter and another through which they could exit.” But he chose to forego that which he thought was better, so as not to upset the people. 

End quote. 

He also said in al-Fataawa al-Kubra (2/320): 

If the imam does not do something that the person praying behind him thinks is obligatory there are two opinions concerning that. The more correct of them is that the prayer of the person praying behind the imam is valid. This is the view of Maalik and the clearer of the two reports narrated from Ahmad concerning such matters. This is also narrated from al-Shaafa’i, because he used to pray behind the Maalikis who do not recite the Basmalah, but his view is that it is obligatory to recite it. The evidence for that is the report narrated by al-Bukhaari and others, that the Prophet (peace and blessings of Allaah be upon him) said: “They lead you in prayer and if they get it right, it is for you and for them, and if they make a mistake it is for you and against them.” So he said that the imam’s mistake counts against him but not against the one who prays behind him. So if the imam’s view is the correct one in such cases, then there is no dispute; if he is mistaken then his mistake affects him alone. The one who thinks that the person praying behind the imam should not follow him in that says: the person praying behind the imam thinks that his imam’s prayer is invalid. Shaykh al-Islam said: That is not the case, rather he thinks that the imam is praying on the basis of ijtihaad or taqleed; if he gets it right then he will have two rewards and if he gets it wrong he will still have one reward. 

End quote. 

And he said in al-Ikhtiyaaraat al-Fiqhiyyah (70): 

If the imam does something that is forbidden in the eyes of the one who is praying behind him but not in his own eyes, which is a matter that is subject to ijtihaad, then his prayer offered behind him is valid. This is the well-known view narrated from Ahmad. And he said: The reports narrated from Ahmad may appear contradictory but they are not, rather the apparent meaning is that in every case where a person is certain that the imam has made a mistake,  then he has to repeat the prayer, and in every case  where there is no such certainty, then he should not repeat the prayer. This is what is indicated by the Sunnah and the reports. There is a well-known dispute on this point among the scholars. 

So you have to follow the imam, because this is a matter of ijtihaad concerning which the imams differed. 

If you can advise the imam and explain the Sunnah to him, with evidence, then this is what you should do. 

And Allaah knows best.