DS (18 months) doesn’t say much.
He used to say ‘mum/mama’ but seems to have stopped, although he says it to his dad and at nursery sometimes. He says Hiya a lot, daddy, baby/baba. He does say Cat, but it isn’t clear and sounds like Da (but different to when he says dad, I know this makes no sense but you can sort of tell!) Tractor - dah-doooor, and Nigh (no.)
So - I’m trying to encourage a bit more as I keep reading he should have a vocabulary of between 15-20 words and am worried we’re a bit behind. He has loads of books and I read with him and try to teach duck, star, car, shoes, etc. But he doesn’t seem very interested and although he will read with me he’ll only engage with books that he can interact with, so touch-feel books, or lift the flap, or sounds. He gets very annoyed if you just sit and read 
Any good ideas?