I'm surprised your doctor didn't tell you things you can do yourself at home!
When I took my son to his 18 mo. check up, I was a little worried about his speech. The doctor said that we usually don't immediately think of all the words our child knows, and sent me home to write down all the words he said for a few days. He said I'd be surprised, and I was.
The other thing he said was to try a "game" with him, to see if he was capable of learning words. The most important part of this "game" is to not frustrate the child. Here's what you do. Before you hand your child something, hold it out of reach and say its name (for example, "cracker" or "truck"). Say it about 3 times and see if your child will try to say it. Reward an attempt with praise and by giving him the object. If your child shows frustration, just give him the object and try later. My doctor said that if my son didn't learn any new words that way, then we'd look for a problem.
A HUGE thing that relieved my worry was when the doctor said that animal sounds count as words in his vocabulary. So count those, or teach him those with fun books, wooden puzzles, Old MacDonald Had a Farm, etc. My son knew all the normal animals, and then some, so that really boosted the number of "words" he knew.
After you do all of that, if you're still worried, then I would look into programs for him. But I think there is SO much you can do on your own, at home, especially now that you know there isn't a hearing problem!
(FYI, my BIL had a hearing problem, caused by sinus problems and ear infections, when he was young. He only said "uh" for truck by the time he was 3, and has problems still at age 17 because he lost some valuable years. So the fact that your son doesn't have hearing issues is VERY good!)