I have a 4 year old girl, who is similiar to your son. They call this speech delay, and it's nothing you did or didn't do. My daughter is gradually learning more and more, and I believe she will eventually be fully verbal, but right now, there is a marked difference between her and her peers, and it's embarrassing to me when other adults address her and she can't respond. We have a 21 month old son, and he is progressing normally, and it's refreshing to me, but also confirms that my daughters problems are not attributed to my bad parenting.
I would seek the help that's offered. We had lived in Oregon last year and my daughter qualified for a special ed preschool. They knew how to help. We've moved away from there, so we continue to work with her in a different setting. If you are a working mom and single, you don't have the option to do home therapy. Start now, and see what can be done. The state is free, but it takes a long time to make it threw the red tape. But, their goal is to get children like this up to speed by the time the reach kindergarten.
You have to grieve the fact that your son is not like other 18 month olds, and once you've accepted that, you must seek help. It is nothing you've done wrong, some children just have a harder time grasping language.
One other thing you can check out is a program called "the Listening Ear"- google it. We're trying it, and yet it's too early to be able to give a conclusive review.
I pray you find the answers you need, and that God will bless your little guy with a tongue that can bless you with the word, "Mama". My daughter was delayed, but she did eventually say it, and it was precious!
M.