M.K.
Sounds just like what we went thru with our son. I could always understand him but others could not. Our doctor suggested speech therapy. Thru the public school he started attending a half-day pre-school specifically for speech therapy just before turning five. He loved it!!! They gave us suggestions on how to help, monitored his progress and it was a great experience as we all worked together. In all, I think he had four years of speech therapy thru his schools.
It was hard for me sending him to school since he was my first baby but I knew it was for his own good. Now I look back on family videos and wonder how on earth I understood him. It was like he had his own language.
I would check with the school you intend to send him to and see if they have some kind of program. It was a great help financially doing it this way rather than go out on our own and have to pay for a therapist.
Good luck!!!!