As a School Psychologist I often would recommend to parents of premature babies who demonstrate some learning lags to consult with their local School Board office and regional early intervention teams. A simple call to your local school should provide all of this information.
As far as home strategies, consider exposing your child to small groups with adult supervised activities, such as library reading times, community summer camps and after-school activities. Exposure to language models may help enhance your own child's skills.
If you are able to contact a Speech and Language Therapist, she/he will be able to offer you specific instructions on how to help your son. A Speech and Language therapist should also be available at your public school that your son may be attending.