It could be partially the needle size, or more importantly the injection technique because he says he has a bruise there now. If I'm not paying attention and pull out too soon as I am injecting (insulin dependent diabetic) I can get an underskin abcess where the substance is stuck in the skin area. This WILL burn as well, even with insulin. So imagine with BAC water. If he's not lean, he could have not gone deep enough depening on his technique and the sheer size of the injection since he reconstituted to 10ml. However, if done correctly, sub-q injections are completely painless as there is zero tissue displacement.