I have a beginning (about 2 months) 6 year-old student who's right hand is usually collapsed at the bridge. Not his left - only his right.

I am used to dealing with other common problems such as collapsed finger joints or excessive tension, but I haven't run into this too much.

When he simply rests his hand gently on the keys, it is fine, but whenever he begins to play, the bridge collapses again. I tried to show him the difference in his hands, but he just can't seem to hold up that right hand.

Do you see this very often? When you do, what are some tricks you use for correcting it?

Thank you!

Pieces involving clusters can help, as well as "pecking hen" pieces and exercises. Also realize it's one of those things that will take a while to develop, so expect it to take several months for better habits to be established.

What on earth is a "pecking hen" piece?

This, perchance?
Pecking Rooster/Hen

When I read the title of your post, I thought the Mackinac Bridge was falling down!

PS This little joke is only something fellow Michiganders will get.

Yes!

Unless you're already all set, here's a simple solution that has worked with a few students...

I just had them gently (without squeezing) hold a ball in their hand (tennis ball is usually a good size) in such a way it got their fingers nice and rounded. As I removed it slowly I'd ask them to freeze their fingers in that position and place them on the keys.

This is of course similar to the PA suggestion to place your hand on your knee to get rounded fingers - but, the difference being, I can say "now when you play, pretend the ball still needs to fit in your hand". This little added visualization seems to help, and it worked with the few I tried it with.