15 months still not walking?

My son is almost 16 months and still not walking. He was born a month early and was in the NICU for a month due to breathing and eating issues. He seems to be very healthy now and has a hearty appetite. He crawled before he turned one. He still crawls and is very good at it. He can pull himself up on furniture and actually like to go to doors stand against them and open shut them. He does stand on his tippy toes most of the time which concerns me. We saw his pediatrician a few weeks ago and she seemed concerned telling us it could be cerebral palsy. Ever since I have been stressing out. He seems normal and healthy. He cruises along furniture and even climbs stairs like a pro. I try to hold his hands and walk but again he uses his tippy toes. He has taken a few steps by himself but will just fall to his buttons. I practice a lot with him. Somedays he will cooperate and try. Somedays he just doesn't care and just crawls as if he has no interest. He seems very strong just no Interest and maybe hard time balancing. He can also walk pretty good with a push toy. Again he only does it occasionally. He is a busy body and yet I'm still worried about his walking. I'm worried he will be confined to a wheel chair for the rest of his life. Has anyone else ever experienced a late Walker? Should I be concerned or over thinking also I am guilty of over using and excersaucer in the past. I do let him crawl everywhere now though.