Bless him. I had this as an adult and it is horrid, horrid, horrid.
Your pharmacist should be able to advise on this (mine was much more use than my doctor in OKing things to treat the spots, which can go into any orrifice on the body. Horrid!!)
I found that cutting grapes in half and then mixing them with a green jelly, set as normal in the fridge, was fantastically soothing for the spots inside my throat - they may help his mouth too. The grapes were much cooler than the jelly - perhaps just keeping your grapes in the fridge would help too.
I also found that piriton helped, though again, check with the pharmacist for a little one, expecially if he is taking anything else. It reduced some of the itching and, of course, helped me sleep much more too.
How far in is he? (Chicken pox spots arrive in three waves, often affecting three different areas of the body. If I remember rightly the waves come 3 days apart. I had spots from head to toe and looked like something from Dr Who!!) Big hug for your ds.