Encouraging my son to speak

I am new to this forum and if anyone can help me through this maze of suggestions and processes I would be really grateful.  My autistic  3 year old son has had a diagnosis and  I am at a loss as to what I can do as a parent to encourage him to speak more.  His sentences consist of mostly the thing he wants with the occasional "please".  He seems to understand much more but how much more I cannot say.

At present we are trying massage, the Gemini programme and I have begun intensive interaction on the advice of the SLT.  Nothing much has changed, maybe I am expecting too much too soon.  But its very hard to know what to do.

Any advice would be appreciated