Hi I don't know if your still looking for an answer but here is some info that might help.
My son has been on insulin for 15 months now and he injects into his stomach, he used to say it was burning and hurt, then we remembered a small bit of advice were were told at the start.
you should be doing it like this...
grab a bit from a fatty part of the stomach
insert needle in straight (not on an angle)
Inject the required dose
LEAVE THE NEEDLE IN FOR 10 SECONDS AFTER THE INJECTION IS DONE.
This was the problem for him, the reason is, if you take the needle out too soon, small drops of insulin still come off the needle and onto the skin, this can cause stinging and redness round the injection site.
My son also uses novorapid and levamere ( I have just started on both of them too, and I know the burning feeling, also if you inject into the same site that can hurt too as the site might not be recovered from the last time you injected there. thats why you should move sites around the body.
My son is now 14 and he in on 6mm needles, I am on 8mm needles but I have tried his and 6mm is much better, so your son might be better off with 6mm (if he's not on them allready) eventualy we all have to go up to 8mm anyway.
I hope this info is usefull, it's only the obvious, but sometimes people forget the obvious.
Good luck and hope this helps.