He has a piece of land which he is keeping in case he needs it; does he have to sell it in order to do Hajj?

Hajj is obligatory for one who is able to do it, because Allah, may He be exalted, says (interpretation of the meaning):

{And [due] to Allah from the people is a pilgrimage to the House – for whoever is able to find thereto a way} [Al ‘Imran 3:97].

What is meant by being able to find thereto a way is being able to do it, both physically and financially. What is meant by being able financially to do it is having enough wealth to cover the expenses of Hajj, namely the journey there and back, and these funds should be surplus to his basic needs and the needs of his dependents for food, drink, clothing and transportation.

If this land is not cultivated to grow food and is not rented out so you can live on its rent, and you do not have any debts for which you need to sell off the land to pay them off, then it is surplus to your needs and you must sell it in order to do Hajj.

Ibn Qudamah (may Allah have mercy on him) said: What matters is that [this wealth] should be surplus to what he needs to spend on his dependents on whose maintenance he is obliged to spend, from when he leaves until he comes back, because spending [on their maintenance] has to do with people’s rights, and they are more in in need of the money and their rights are more important. ‘Abdullah ibn ‘Amr narrated from the Prophet (blessings and peace of Allah be upon him) that he said: “It is sufficient sin for a man to neglect those for whose maintenance he is responsible.” Narrated by Abu Dawud.

It should also be surplus to what he and his family need of shelter, a servant and other essentials, and should be surplus to what he needs to pay off his debt, because paying off debt is an essential need, and the rights of other people are connected to it. Therefore it is more important, hence the Lawgiver disallowed giving zakah [when there is a debt to be paid off], even though it has to do with the rights of the poor and they need it. So it is more appropriate to defer Hajj, which is due only to Allah, may He be exalted. It is the same whether the debt is owed to a specific person or is something that is owed to Allah, may He be exalted, such as zakah that is owed, or expiation that is to be offered, and the like."(Al-Mughni  3/88).

The Permanent Committee (11/30) said: Being able to find a way to do Hajj means being physically healthy and owning the means of transportation to reach the Sacred House of Allah by plane, car or mount, or being able to pay for that according to one’s means, as well as having sufficient provision for the journey there and back. These funds should be surplus to what he needs to spend on those on whom he is obliged to spend until he returns from Hajj. A woman should also have her husband or mahram with her, even to travel for Hajj or ‘umrah. End quote.

The fact that you are keeping this land in order to marry off your son, or for other possible emergencies, such as needing medical treatment and the like, is not an excuse for not doing Hajj, so long as you could afford to do it with the proceeds of the land if you sell it. So put your trust in Allah, and do what Allah has enjoined you to do. Think positively of Him and seek compensation from Him.

It was narrated that ‘Abdullah ibn Mas‘ud said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Follow one Hajj or ‘umrah with another, for they eliminate poverty and sins as the bellows eliminate the dross of iron, gold and silver.” Narrated by at-Tirmidhi (810). He said: It is a sahih hasan hadith.

It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “There is no day on which people wake up, but two angels come down and one of them says: ‘O Allah, give more to the one who spends,’ and the other says, ‘O Allah, send destruction upon the one who withholds.” Narrated by al-Bukhari (1442) and Muslim (1010).

And Allah knows best.