These stories of delayed children later excelling are wonderful! My son was developing really well and I wasn't one to worry. In hindsight there were little flags like the ones you've raised. He never waved and did all that baby social communication stuff... and now he has a severe social communication delay/possible autism. I wish I had spent less time leaving him to make up his own milestones and had just taken him to see a professional and have him assessed.

I'm not sure your son has enough 'red flags' at this point to be concerned, but personally I would get an assessment at 18 months if you have ANY doubts still.