So far we have:
Rafi (ay ay... at least we think he means Raf)
duck (du... or ca ca - quack quack)
go go (also stands in for choo choo)
There are other things, but we're notoriously bad when it comes to interpreting things. My mum reports that he's been saying lots to her. We were singing some songs in the car and he was happily singing along, too!