It sounds like you're doing all the right things. Just keep it up! Every child starts talking at their own pace. 15 months without any words is a bit behind the milestone (2-3 words by 1 year is the normal milestone), but missing a milestone is absolutely no cause for worry. It is only when you see a pattern of missing multiple milestones that you should really start to be concerned. Your pediatrician should be asking about language development at well-baby visits, and will let you know if there is reason to be concerned.
I believe the most important thing is to talk to him as much as possible. Continue with the running commentary of what you're doing. Supplement it with asking him questions, even if he doesn't answer. The more you talk to him, and in front of him, the more he'll pick up, even if he's just passively learning it.
Instead of pointing at things and saying what they are, try pointing to them, and asking your son what they are: "What's that? Is that a cow? Can you say cow?" However, don't pressure him. Make it fun. The more it seems like a game to him, the more likely he will be to participate. Smiling while asking the question is just as important as your tone of voice.
The other thing to do is read more. I am now convinced that you can't read to a child too often. If your son enjoys sitting with you and looking at the books, even if he isn't pointing and identifying pictures, it is building his vocabulary for when he is ready to talk.