I have no recommendation for a kids full face (I actually prefer an open face helmet for any kind of jumping as it doesn't restrict view when spotting your landing like a full face does) but can see why you'd want a full-face for him. but can I offer a wee bit of advice for learning to jump? fit a seat QR to his bike and get him into the habit of slamming his saddle as low as it'll go the reason I say this is when jumping goes wrong a high saddle will often hinder a rider rolling out or jumping clear and can actually catapult you into a face plant. I'd actually try and spend some time teaching him how to bail too if you can.
Tj – how on earth do you have experience of kids full face helmets? 😕
<EDIT> I do have a little advice on helmet choice, you need to try a few on him and you want the tightest fit without being uncomfortable that you can get. fit the helmet, do up the buckle and hold the chinguard and move it up down side to side, you want the one that moves the least. a full face that rotates up easily when you hit the chinguard might as well not have one.