Congratulations on your beautiful little boy. So sorry to hear he's had a rough start. I'm a pediatric physical therapist and very familiar with hypertonia in babies/kids. Hypertonia in babies can be caused by lots of things. Since your son was born so early, it could be that he had or has some damage to his brain, commonly known as cerebral palsy, but I imagine he spent some time in the NICU and they have probably run tests to watch for that. HOWEVER, hypertonia can also be caused by babies over-using some of their muscles, often if they have reflux or some other minor medical problem that causes them to be painful in certain positions. Very often, if you can get some treatment (which should be free up until age 3), you can avoid any long-term problems and help him to grow properly. Babies are wonderfully able to heal and change!
Unfortunately, there are times when the brain has been damaged a little at birth, and if that's the case there are still lots of things a therapist can do and can teach you to do to help. The important thing is to get into the "system" as soon as you can.
You can start in a few places. You can call your pediatrician for a referral to your local birth-3 program, and also if they think it would be good to see a pediatric neurologist or physiatrist about the hypertonia. You can contact your local health department and ask them directly who to contact for an evaluation. Usually the birth-3 program is run through them anyay. You can call a local therapy center and speak to a pediatric (physical or occupational) therapist. If you aren't satisfied with the path you choose, try another one. It's very worth it to see a pediatric specialist if you can, even if you have to drive to a bigger town. In larger towns and cities, most pediatric therapists try to come to your house, but in smaller towns they are often too busy to drive, so you may have to go to them.
Please feel free to ask if you have any more specific questions. I hope your little guy works out of his hypertonia. He's fortunate to have such a caring parent!