Where exactly is the second lump and are both lumps movable and not tender or red? Why was he giving the anitibiotic--what infection was being treated?
OK--the ones in the back of the head could be lymph nodes, but not the one near the ribcage--nodes aren't in that area. These could rather be sebaceous cysts that may or may not be infected, or they could be small boils--a skin infection. If they continue to enlarge and don't drain, they would have to be lanced to treat them, but a second course of antibiotics like Septra DS might work for them. Of course, if the ones in the back of the head are nodes, he might have mononucleosis or--and I don't think this is very likely--lymphoma. I would have him evaluated by a pediatrician at this point.