M.C.
Definitely take him to a doctor! My cousin had a "boil" like this near his scrotum when he was about 4 and by the time his mom got him into the doctors office he was getting sick from infection. They were worried he might end up in the hospital. If the infection gets into the blood stream kids can get SO sick, really fast, but if they catch these things early they can be treated and fixed easily. Plus, they may be able to drain it... which will make him feel so much better.
Good Luck!
-M.