I want to tell my girlfriend's 11 year-old brother that I would pay for his college fund if he stays in school and gets good grades, even though I don't have the money right now
Why are you offering to pay for your gf's brother's college fund?
Because he asked me for money. I know if I was a good person I would help him out, but I don't really have the money.
I understand that you want to help out your gf's brother with his college money, but if you cannot afford it, it's irresponsible to make the offer. Find the financial resources you need before you commit to helping out on a college fund.
I guess I over-promised what I could deliver.
It's admirable that you'd offer to pay for his education, but not if you see no way of having the money when he'll need it. If you think you might be in a position to help down the road, you need to explain that to him so he's not counting on it.