My 2.5 year old can but is not interested in talking? What to do ?

I know he can say words because he says Mama alot but compared to kids his age his speech is very limited. I try to encourage him but he is just not interested. I can tell he understands everything so he does not lack comprehension. He also makes alot of normal kid noises so he is not a mute either. The doctor and his carers at school mention it. What do you do if your child is just not interested in talking. I am thinking maybe he is just an introvert or just lazy.