Yes, my son had an MRI when he was just about the same age -- to see if the head bones were growing, or had they already closed off. I think they are called sutures. We both survived ... Tony did better than I did! He was given some oral liquid medicine to make him drowsy -- it's better to do it right than have to repeat it or have the doctor say, "Let's wait until he's a little older" and if there was a problem....
I'm sure they will send you to a Pediatric Neurologist (is that the specalist?) to help with it -- maybe not.
Frankly, though, although all children progress at their own rate of speed, it appears your son needs some extra help. Better to do it now, see what needs to be done, and then, because small children are like sponges, if he does need a bit more help, then you can go down that path. We had a Physicial Therapist, Occupational Therapist, -- it seemed just about everyone helping with my son -- born at 29 weeks and didn't come home for 3.5 months.
(This was a long time ago.... did he catch up? Yes, he did, but with lots of intervention -- he never was good at eye/hand co-ordination until he started playing video games and we used a balloon to practice catching rather than a ball..... it comes down slowly and it's big enough to see! I tried to teach him how to crawl -- once someone watched me and told me I had my legs and arms wrong, it went a lot faster!)
You and your son will be fine. It's scary -- but physicians don't do things because they need more practice or because they want to.... if he suggested it, then do it!
God bless you, your husband and THE triplets!