Wow! This sounds exactly like my 3 year old daughter. She did not walk until 25 months. Still cannot jump and stairs a big problem. Her limp came on a couple weeks ago. We've been to a pediatric orthopedic specialist and two ERs. CT scans, x-rays, etc. etc. The last ER doctor told me it sure does seem like a Cerebral Palsy or neurologic limp and not an orthopedic limp. We are following up with a neurologist tomorrow. I would suggest a neurologist. Nobody wants to hear the words cerebral palsy but there are huge varying degrees to this and it won't kill them. So if the worst thing is physical therapy and an on-going condition then there is much hope in that. Good luck to you! My prayers are with your precious son.