I can understand your worries regarding injury to tip of penis, which as you rightly said is a very sensitive area. The good thing is that there is no sign of scratch now and there is no pain. This means that the injury has healed itself. It is very unlikely that there will be any injury to nerves.
Injury to nerves is expected if there was lingering cut or infection or there was loss of sensation on the penis tip. In rare cases there may be a loss of proper erection. So, if your son is not feeling ay pain and the tip of penis has sensations then nerve injury is highly unlikely. However, please discuss your concerns with his doctor as well. Take care!
The nerve damage that this scratch may have caused is nothing compared to the nerve damage that circumcision certainly always causes. Too late for your son, but anyone who wants to avoid any nerve damage in this, as you say, "very delicate area" always has the option of not circumcising. That is what 3/4 of parents in the world do, and their sons suffer no ill-effects.