My son is 2 yr 10 months old & he is not properly talking. He speaks only single words. Can you suggest me what should I do?

My son was born preemie (27 weeker)he has a speech delay and so his pediatrician referred us to a speech therapist and he is also attending a specialized pre k class 4 times a week.His speech started progressing when we enrolled him to pre k special program..also my son tends to learn more with visuals (based on his teacher during his evaluations) so we let him use ipad to watch kid shows (under limited hrs and supervision).Now he can talk straight sentences..Just a reminder not all kids are the same though so the best recommendation is to go to your pediatrician first.