How can he complete what he starts?

The problem cannot be solved through advice and preaching; rather what you are suffering from is a behavioural problem, which requires a practical solution and remedy, and mere words of exhortation and verbal advice will not be sufficient.

The first thing you should do to overcome this shortcoming is clearly to persist until you complete the deed. Probably the first successful experience will motivate you to keep going until you complete all your deeds and do them perfectly. What will help you to achieve that is two things:

1. Dividing the deed into small steps, so that you will feel more motivated to complete each stage on its own, and keep going until you reach the last stage and complete it. That is because human nature finds large, lengthy tasks burdensome. So there is no alternative but to break the task down into stages until you achieve it.

2. Choosing short and quick tasks and beginning with them, so that you will be able to complete them. The Prophet (blessings and peace of Allah be upon him) said: “O people, you should only do deeds that you are able for, for Allah does not grow weary but you do. The most beloved of deeds to Allah is that which is done persistently, even if it is little.” Narrated by al-Bukhaari (5861) and Muslim (782). And he (blessings and peace of Allah be upon him) said: “The most beloved of actions to Allah are those which are done persistently, even if they are little.” Narrated by al-Bukhaari (5861) and Muslim (783). 

Imam an-Nawawi (may Allah have mercy on him) said: In other words, that which you will be able to do regularly without it causing you any harm. This indicates that it is encouraged to be moderate in doing acts of worship and to avoid taking on too much. The hadith is not speaking only of prayer; rather it is general in meaning and applies to all righteous deeds. End quote.

Sharh an-Nawawi ‘ala Muslim (6/70-71).

We advise you to read some books and psychological studies, which you can benefit from regarding this matter. Such books include: al-Futoor, Asbaabuhu wa ‘Ilaajuhu by Shaykh Dr. Naasir al-‘Umar, and ‘Ajz ath-Thiqaat by Shaykh Dr. Muhammad Moosa ash-Shareef.

And Allah knows best.