Okay, coming in late to this thread, but wanted to add my experience. DS had had *numerous* ear infections and was also not saying any words by his 18-month checkup. The combination of those 2 facts made the pediatrician refer us to Early Intervention. He had a hearing test and failed because of fluid in the ears. They said that if the fluid had been present all along (which it likely had been) then he hadn't been hearing sounds normally and therefore couldn't reproduce them. He had tubes put in, he was evaluated by Early Intervention and qualified for speech therapy. We started therapy, he said his first word at 21 months, and by his 2nd birthday had a huge language explosion and hasn't stopped talking for a second since then. So while I don't think the therapy is what got him talking (I think the tubes were more responsible), I agree as others have said, it's absolutely worth having his hearing checked, and IMO, an evaluation by EI.
ETA: You mentioned your son understands lots of words and can follow directions. From what I've been told, that's an excellent sign that all is functioning properly. At DS's evaluation at 18 months, he tested at 26 months for understanding language (I think it's called receptive language?), his spoken language tested at 9 months.
"There are times when we're dirt broke, hungry, and freezing, and I ask myself, why the hell am I still living here? And then they call. And I remember." ~Mark, Rent