Both of my children were late walkers. My daughter at 16 months and pretty much had no interest in putting weight on her legs. She was not born early or anything - just content with crawling apparently. She is fine. The Pediatrician did mention that he would suggest PT if it went past 17 months, but that was because she didn't put any weight on her legs in his presence.
My son walked at 17 months!! Imagine our surprise because he was pulling himself up at 10 months (seriously). We discovered that my daughter was pretty much doing everything for him - he had no motivation. Once we managed to make it a game with our daughter to get him to walk, he figured it out because he maid was no longer doing everything for him (LOL).
I'm not sure I have much advice, but I can tell you some of the things we tried. One thing that worked for us was motivation, esp. with my daughter. I also had one of those riders that turned into something they could hold on to and walk assisted. That helped my son keep up. I just practiced a lot with my daughter.
If your son was born early and is already a late bloomer, then I think your instincts are probably correct. Ask yourself - is he cruising? Does he put weight on his legs? Could he walk, but is choosing not to? If yes to these, then you are probably right. However, a friend of mine had an early baby too and she has been in PT for the better part of a year and a half because she has some issues with walking. It has helped their daughter, so if it's something you can afford, perhaps it's not a bad thing. Good luck.