My DS-now-6 was a late talker although not quite as late as your DS2.5. He too had letters, numbers and shapes among his early words. How long has your DS been using a substantial number or words? In my DS's case, at his 22-month checkup he had fewer than 5 words that he used regularly, but then as he turned 2 I had just given up recording his words because he had well over 100 and I couldn't keep up, and by 26 months he was talking in 4-5 word sentences. He never stopped, and from then on nobody would guess (or believe, often) that he'd been speech delayed. The health visitor who had done his 22-month checkup had guessed that it would happen like this, so had told me to call in 6 months if he hadn't had a language explosion by then; so no therapy experience to share.
One thing is, DS never actually seemed to have any trouble communicating. We had our "he understands every word we say" (i.e. even when we're not talking to him) realisation when he was around 15 months, and much earlier than that he would understand anything said to him. We did a bit of sign language, and I suppose a lot of gesture interpretation!
