My preemie son had similar symptoms, except his eye contact was problematic and he would go off in his own world - I would wave my hand in front of his face at 14 months old, no reaction. Cerebral palsy was ruled out first. We did do the MRI, and extensive genetic testing to rule out fragile X, etc.
He was diagnosed with global developmental delays, and had intensive physical, occupational, speech and "play therapy"! Yes, apparently play therapy is a thing. PLEASE DO ALL THESE THINGS, OP, THEY WILL SAVE YOUR SON. And don't just do the weekly or biweekly therapies. Take notes, and do practice sessions yourself at home several times a day, every day! Buy the material, or make it at home, whatever you need to do. Early intervention and intensive practice is critical. The speech method I used at home with him was "It Takes Two To Talk". You can buy it on Amazon.
One day the physical therapist and I were chatting and we realized 14 month old DS had laid out all the little sensory balls she had brought in order of size and color, in a grid. He had no speech but could pronounce in a recognizable way all the brands of cars whose logo he saw passing by. My father printed them all out and spread them on the carpet, and even upside down, this toddler could recognize them instantly. We suspected high-functioning autism at the time.
Years later, he was diagnosed with inattentive ADHD (the daydreaming kind, symptoms were clear from the beginning!), and some learning disorders associated with it, like dyscalculia (specific issue in math). His IQ is actually in the low part of the gifted range, at 130. He is a high schooler now, taking all advanced classes, with an extra time accommodation for tests and special counseling services to keep him organized.
Jury is still out on the high-functioning autism. We didn't get the testing for that, because he was getting all the services and accommodations he needed with his ADHD label at school. He is introverted and not very social, but that's fine - we're all like that in the family
Best of luck, OP. You have your work cut out for you, but if I could do it, so can you!