I wouldn't restrict fluids before you have a bunch of tests done. If he does have diabetes insipidus, and it sounds certainly like he does, he will dehydrate VERY quickly and become a medical emergency.
Children with diabetes don't pee so much because they drink too much; they drink so much because they pee too much.
I would call the specialist back again, and make it clear how much he is drinking and peeing -
Best wishes.
I agree with your Husband, it is a habit but also he is asking for attention,I think out of worry you are reading more into this,however if you are cocerned it doesnt hurt to check it out. Do you offer the Drinks to him at all or does he always ask for them,perhaps some distraction and reading and Games could work to cut it back.Dont give him a drink before Bed he doesnt need it it is a Habit,and possibly a way of seeking attention from you, give him the attention but cut back the drinks .