J.
Hi A.
My son had the same issue. He was also premature - born at 29 weeks. I took him to a therapist (where they do the follow up visits for St John's) anyway, this is what they did and it worked like magic! You have to teach your son to chew. You will want to take dry cereal, puffs, crackers, etc and crunch it up into crumbs. You will take your finger, put some crumbs on it and rub it against his gums (in the back, on the bottom) and tell him to chew, chew (you will want to make the motion with your mouth as well so he can see you do it).
Soon you can make the crumbs bigger. It also helps for you to give him those plastic baby tooth brush (gum brush) things for him to chew on.
My son was close to a year and only on soft food so that is when I saught out advise. He was quick to learn to chew and then he could not get enough especially of the baby puffs - these are a little easier since they melt in the mouth.
Good luck!