at 2.5 years my eldest had less than 10 words, and could not string any together to make a sentence. We took him to a chiropractor (about his knees, not his speech) but they did something to his neck and he started talking that day with heaps of new words and sentences within a week. They explained it as the nerves that control speech were being constricted because of a part of his neck being out of alignment, and getting the alignment right again freed the nerves to fire as they were intended to. He still had some speech therapy to help him catch up but now aged 6 he is completely normal speech wise.

Anyway my point is, try a chiropractor and see if they can help, in conjunction with keeping an eye on his hearing and perhaps some speech therapy. We also found it helpful to use sign language both to aid communication and to increase his understanding of the consequence of speech and it's outcome. Good luck