I'm thinking of recommending a classmate wear bras in class
Do you think he's actually sprouting breasts?
No, but I think he has a chest problem.
He's probably aware of the situation if it's evident to you. Commenting on the obvious will only make him more self-conscious. If you're going to approach him about it, at least say something constructive.
My suggestion is perfect so what are you saying?
You are just trying to make him feel bad. There's no need to say anything like that.