Speech Delay

I have a 20 month old son who was born premature (33 weeks gestation). He has had a delay in achieving his milestones. He continues to undergo physiotherapy and started walking when he was 18 months old. He used to make babbling sounds when he was 1 year old and started speaking one or two words like mama, dada when he was 13 months old.

However he has not moved beyond those two words and till date has not added any new words to his vocabulary. He is attentive, active and extremely intelligent in understanding instructions. He has mild stranger anxiety, but is a completely different person around his family. He coveys everything in gestures and is very cheerful and active. He is able to convey his likes and dislikes with gestures. We are worried over his speech delay. Can this be related to autism or is it normaly delay due to his prematurity?

We are getting worried over