He'll remember how when he gets mad enough !
I know it's tough on you to listen to him cry, but this is one of those things where kids need motivation to do what they need to do. If you help him every time he cries, he has no motivation to do it himself.
There will be MANY times in his life when he is frustrated or just down right mad, and he is going to cry or even scream. Try to remind yourself that he is not hurt, and not in any danger. He is just mad. Try really hard not to step in unless he really does need some help. You don't want to interfere with his learning to do things for himself, or his sense of pride when he accomplishes something all by himself.
In this case .... he did it before ? He'll do it again when his motivation is strong enough. When he does, PRAISE him for it. Make a big deal about what a big boy he is, and how proud you are. Even a 7 month old can understand your pride in your voice, and it is very affirming for him. He'll think you're pretty great, too !
Best wishes !