Spider bites can cause numbness and welts. Spiders usually tend to bite people in their sleep for some reason. The concern I have is if it was a brown recluse or black widow? These bites are far more dangerous than most common spider bites. I don't know where you live, but the best thing to due is look up images of brown recluse and black widow bites on line and compare them. I doubt this is the kind of spider bite that your son has, but without looking at it, I can not say for sure. If you determine that it is not one of these two bites, then try applying hydro cortisone cream and giving him benadryl. Make sure to keep a watch for infection and anything else that doesn't appear to look right. I hope this helps you.