If he's been playing for a while and still has this problem, I'd hazard to guess that when he goes down to his knees he's dropping forwards, as opposed to down. If you have your skates together and fall forwards to your knees, such that your skates will be together behind you and your pads are sitting on their faces, you're in a much more awkward position, and keeping your balance will be a bit difficult.
If he wants to be in the traditional butterfly position, he probably needs to do two things. 1) keep a wider stance. The skates should be at about shoulder width apart. 2) Perhaps loosen the straps on the leg pads. If they are too tight, it will prevent them from rotating around his legs as he falls to the ice. This is why leg pads need to have adequate knee protection, to cushion the knee and prevent it from slamming on the ice (which hurts really bad. I speak from experience). If he does this, with practice, he should be able to come into a stable butterfly position on the ice. Once he gets acquainted with it, he shouldn't be tipping forward any more.
Also, one thing that I did when I first started playing was hopping up to get into the butterfly. Ideally, you should just collapse down. Much time is wasted by hopping up, just to get down. Think of it this way, say you're standing on the ice in a crouched position. You roll your ankles so that you're on the inside edges and the boot of skate is starting to touch the ice. If you let them, the skates will slide on the boot, away from each other, and you'd come to rest on your knees with your skates splayed apart at your sides. That is pretty much the exact motion I use to get into a butterfly.
Now, after having said all that, I'm no coach. So take this is simple advice, not instruction. It's up to your son to figure out what works best for him. Time and practice will help.