My husband seems to always get an abcess...or infection, after having a crown. Our dentist got to where he left the temp crown on a little longer so that if the infection set in, he could do the work before putting the permanant crown on. If it is an infection, he'll have to take some antibiotics to get rid of it. Then, they do a root canal, which is basically taking out the bad part of the root...sounds awful, but hubby said it's really not as big a deal as people think.
Anyway, I think I would watch it and if it doesn't go down before long, take him back or find another dentist. My crown didn't give me any trouble at all!!
Good luck!