That's the max that I can afford at the moment (I'm 22) but I want to save it on a wllet of paper and give it to my sister telling her to keep it safe and that we will give it to him when he is 18 years old. Do you think is a nice idea?

whatever you want.
I think you can give him what he needs for now.
and it would be nice if this discussion is more on productive things that build or share knowledge.