Hi Helen,
I remember the place you are now with your son, the emotions you go through, the utter confusion, i was there 3yrs ago with my son, then aged 7. Things do get easier, as for injection sites, as his only 4, why not try a star sytem chart, when he does it in his leg he gets 1 star on the chart, but if he does it in his tummy he gets 2 stars, aiming for 15 stars or more in a week to get a surprise/treat (comic, kinder egg ect) We did it with Andrew and it worked wonders, and it doesn't have to be expensive, a 97p toy car out of tescos.
Hope he's feeling better today, there's plenty of us Mums here to help you along
Take care
Suzi x