M.P.
Fevers are no fun. It sounds like he may have picked up another virus. 99.9 is a lower grade fever usually associated with a virus. My son had pneumonia, H-flu(conjuctivitus) and a respritory type virus all within about 6 weeks and each one included a fever. We have been illness free for about 1 month now ...YOOHOO!! I would take him in and have his ears, nose, throat and lungs checked just to feel more at ease. I took my son in with each illness...we have a great Dr.
Take care
-M.