This happens from time to time, it simply means you pierced a blood vessel. I hold a tissue over the spot and apply gentle pressure until it stops bleeding which usually doesn't take long at all. I'm sorry that your son is finding the injections difficult. I was lucky that Jess never really had a problem with the injections. Perhaps other parents whose children found it difficult could make suggestions of how you might be able to make it easier for him. How old is your son?