Non verbal means just what it says. If he's not expressing any sounds you can identify as words, that is non verbal.
There is no specific age at which any kid should be talking. 15 months? Are you already suspecting autism? Why? What else are you seeing? What you've got here isn't enough to go on.
Edit: Sorry. Never mind. Took a look at your profile, so now I know. I think.
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I'm not likely to be around much longer. As before when I first signed up here years ago, I'm finding that after a long hiatus, and after only a few days back on here, I'm spending way too much time here again already. So I'm requesting my account be locked, banned or whatever. It's just time. Until then, well, I dunno...