My little brother, currently 7.5 years (I'm 26) was delayed in his speech. My mom had a hard time getting him into a good speech therapist, apparently all the good ones required recomendations and everyone kept telling my mom he was still in the normal development range. She kept at it and finally got him into speech therapy when he was between 3 & 4, not sure as I wasn't living at home anymore. She found out a few years into it that he had been in the 8th percentile when he started speech therapy. We aren't real sure what caused the delay, his hearing is fine. But, when he was a little over 3 his ear drum burst and he hadn't even complained of it hurting, which caused my mom to wonder how many infections he had had over the years. It very well could have been the cause. His speech is improving. If he gets frustrated it gets worse. We can all mostly understand what he is saying. But it is still tough, my 5 year old speaks much better than him. (Which causes plenty of tension between them)
My niece is completely deaf, they aren't sure when she lost her hearing, or if she was born deaf (she passed her initial hearing test, but my sister isn't confident in the nurse that administered it) My niece was a failure to thrive baby and doctors kept dismissing her health problems with, she'll catch up...
My sister knew it was more than that and brought her to doctor after doctor until she found one that would listen to her. She was close to a year old by that time and my sister had already diagnosed her daughter (she did a lot of research) with CMV, a virus that my sister had contracted during pregnancy, although she (my sister) never had symptoms. Blood tests finally confirmed that CMV was the problem and that as suspected my niece was deaf. She had a cochlear inplant by 18 months and with intense therapy is now speaking. Her vocabulary is exploding and most people can understand her. She is 4.5 years and talks about as well as my 3 year old.
Moral of the Story,- You, as the parent knows what is going on better than anyone, so don't be afraid to be persistant until you feel comfortable with the situation.
I wouldn't worry simply because he is 16 months and not talking, even the regression could be because he has found an easier way to get his way, but your concern and worry is enough reason to get him checked out. Hope all works out well for your little guy.