My 9 Year Old Son Was Stung Or Bitten By

My 9 year old son was stung or bitten by an unknown bug/insect on Saturday evening. The top of his foot is still swollen and red today. We did not find what stung him. There is an obvious red dot in the middle of the swollen area. He has always been sensitive to ants, mosquitoes, ect. Is it possible there is a stinger still under his skin that I can not see or feel? Thanks