C.B.
FOLLOW YOUR INSTINCTS!!!!! This is YOUR child, not the doctor's. You (both parents) are your child's ONLY advocate in this world right now. You don't mention whether you have insurance or anything, which greatly affects your next move. Some insurance requires a referral to specialists. If your gut tells you something is really wrong after reading my story, you need to start with deciding how to get your son to a P.O.(like you mentioned). If your insurance requires the referral and his reg. pediatrician won't give it, take him to another pediatrician at least.
You mentioned that you have addressed this problem with the ped. before. If it has been ongoing, my experience may not apply to your situation. At 18 mo. old, my baby girl caught her foot in a chair back at Granny's and limped and walked on tiptoe on that foot for several days, but experienced no pain. My gut told me that she was favoring that leg because she noticed how excited her daddy got about whether that foot was okay at the time of the incident. We decided to check thoroughly for swelling or pain everyday, but otherwise to ignore the limping. Within two or three days she stopped completely and has never done it again. You should trust yourself, and never forget that no one cares about your child's well being as much as you do. Don't panic, calmly go about getting the info you need to parent with confidence.
Sorry so long! Hope this helps.
C.