Agree with pps - let the professionals give the advice. The parents will take it better from them. But you can provide healthy snacks if gs is at your house and maybe some activities that give him some exercise.
Travelsofar, I think you might mention the breathlessness to ds, but say nothing about weight. If the loss of breath concerns him, he'll make the connection. If he doesn't see it, he may, at least, ask the doctor. Otherwise, same advice as to the op. But caution on the activities, in your gd's case, as you don't want her to wear her out or make her dislike visiting you.