I would say that if your speech therapist does not work with kids older than yours, maybe find a different one to be able to start there and then continue as your DS ages out of the original one.
We have had so many SLPs it is crazy and they are all different in ability it seems. I can't give much advice because my little guy is 2e and the best SLPs were involved with the autism school and autism therapy. DS had a different picture than yours with a sky-high "incoming" language but a very low "outgoing" language until ABA and autism-knowledgeable SLPs got to work with him. Once they got him communicating at "brain level" (I just made that up) instead of below age level, then they worked more on his lisp. He still has speech therapy for his lisp (at age 8) and it is slow and steady progress.
When DS was 2, I didn't realize the difference between SLPs, but just like everything some are good and some are not. Unfortunately it is an expensive (money AND time) thing to have to determine by trial and error.
Kate
