my daughter started speech therapy at 18 months, lasted for about 9 months. when she started she had some to none words, and seemed to not be able to understand much. i saw a difference within the first two weeks, not in words but in her command of sounds. i thought it was great. she started with speech therapy twice a week for 30 minutes to once a week for 30 minutes. they tested her every few months and the delay for her age started getting smaller and smaller. after about 9 months she scored higher than her age group. that is when the therapy ended. she is now 4. she has a twin sister. there is a difference between their language abilities, but i cal say freely, had i not taken her to speech therapy god knows where'd she'd be now with language skills.
so, my advice is hang in there. the process is slow, and you should notice widening of sounds and words as time passes. also, if you're not doing so already, leave your son with the therapist while you wait outside. when we first started i stayed in the room with them. she was partly distracted and looked to me for help when she was faced with questions and/or tasks. it was slow at that point. when i suggested to the therapist i stay outside she said i thought you'd never ask. :)
my last comment is: i highly recommend speech therapy, esp. conducted by a person who seems kind, not in a hurry, and gives you feedback afterwards (feedback as in what you should do at home).
good luck