Another vote for early intervention.
We initiated the process around 15 months, started therapy around 18 months. Our son met once a week with a speech therapist for 6 months and it made a world of difference.
I had no intention on waiting to see if this problem just fixed itself. I am a high school teacher and see what happens when parents just wait and the effects, over time, are exponential.
With our kid we weren't trying to turn him into a little Einstein; we just wanted to give him the assistance (which he truly needed) to develop appropriately. Could he have decided on his own to initiate speech at a later age? Maybe - but we weren't interested in waiting, waiting, waiting. We were concerned that he wouldn't progress at a developmentally appropriate rate and took preventative steps to avoid a potential lifelong cycle of special education for an issue that could have easily been resolved.
Where to start - talk with your child's pediatrician who can refer you to early intervention. If your child's ped doesn't refer, you can contact the organization yourself.
Next, they will send a team out to do an evaluation. It won't cost a thing and they'll basically play games with him checking to see his speech, gross motor, fine motor, social/emotional coping skills, etc. This was my son's favorite part of the whole process - hardly stressful! They had blocks, toys, dolls, and lots of little games to play. He had no idea he was being 'evaluated'; he just thought some really fun people had shown up at his house to play with him...with new toys!
After, you'll get a report to see if he is progressing age-appropriately or not. They'll make a recommendation for therapy if one or more areas are of concern. During the whole process, there is a lot of paperwork to submit including financial earnings, which will determine how much you will pay each month for therapy.
Good luck to you. If I were in your shoes, I'd check it out. I'd rather be safe than sorry. And don't worry, you're not being overbearing or trying to push your child beyond genius and making him skittish and less willing to cooperate; you've identified a concern and you're being a responsible parent in trying to help your son!