HI,
I am T1, and have been skiing a few times...although I am much older than 11, and wasn't on a pump at the time.
Is it possible? Yes! Please don't stop him doing fun things becasue he is diabetic. I imagine, that aged 11, you would be with him at all times? He will need to reduce his basal rate etc, and you and he will need to fill you ski jackets up with glucose tablets and facst acting carbs etc.
I wouldn't let him go off on his own...not that you would obviously, but ski school may be difficult..unless you went with him, or the ski school was informed abouthis diabetes in advance, and you could give them a quick pep talk about what to look out re hypos.
I did have a few hypos whilst sking, but as I have goood warnings, I just ate some carbs and got on with it.
I hope that you decide to go, and have a fabulous time! We liked Cervinia on teh Italian Swiss border..lots of wide, easy runs to suit my appalling skiing skills!