If he's not showing any signs that he's interested, just wait. It's great that your nephew is 2 and trained, and that your neighbor's son is, oh wait, she's been training for 15 months? Sorry, it doesn't take that long when they're ready, lol!
They will show interest and if you don't jump on it you'll miss your "window of opportunity," I firmly believe that. My guy was interested around 21 months, I asked his pediatrician who is ultra laid-back, and she said it would put "too much pressure on him" to start training, so I backed off. Then a couple of months later i read in several places that when they show an interest and signs you should go for it.
Well, when we started a few months after the interest he did good for a week or so, then didn't want any part of it, we took a week off, did good, then no interest or cooperation. We took a few weeks off, tried again, he did great a couple of weeks, then decided he didn't want to use the potty at all, grrrrrr. So, we've been waiting and will hopefully be successful in a few weeks, I plan to start again when he turns 2.5. (We are crazy in our house and do a "Half a Birthday" with cupcakes and sing "Half a Birthday to You" to the tune of "Happy Birthday", so I plan to capitalize on the fact that he "Is getting to be such a big boy!" and hope he'll stick with it this time.)
Here's some info to help you decide when to start training and tips for the process:
http://www.parentingscience.com/potty-training-tips.html : )