My 12 yo son gets 5-6 colds each winter. each lasts 3-4 days. low grade fever, soar throat, body aches, strep is neg. should i be concerned?

Not usually: It sounds like these are typical viral colds, that seem to go away in a typical fashion. Although 5-6 does seem to be a lot, exposure to new bugs, like starting a new school, could easily explain this. Also, if he has allergies this may predispose him to getting upper respiratory infections. Finally, make sure noone around him smokes, as this may contribute to frequent infections.

Yes: I wonder if allergies could be playing a role in upsetting his immune system. Try cleaning all the "dust bunnies" under his bed, putting a plastic barrier between mattress and sheets, cleaning or changing the air filters in your heating/cooling system, and washing the cats and dogs with an antibacterial soap. If this doesn't help maybe a visit to an allergist ?