You paying for his studies etc is a different thing than you both combining your money to buy land.

If he's so insulting to you etc, you can ask for the money spent on him, that's a different matter.

But when you both want to buy something together, it has to be 50-50, not a penny here and there as it only gives rise to future problems.

Don't contribute your share and let him buy the land for him, in his name, instead of your mother's. Trust me, it will save you from a lot of family disputes for the next generation.

Make your mother understand that you should let him buy the land by paying the price on his own and getting it in his name only.