He might be a little behind in speech, but he's not yet 2 years old so he's still within the norm or close to it.
Definitely call the speech therapist again. It would be very helpful to have him evaluated so that you have a better idea of what he is doing that you may not realize and so that you know what you can work on. My son was evaluated at about 19 months. He was behind, but there isn't a whole lot a speech therapist can do at that age. She did, however, give me many suggestions, which really helped. It's hard to help him if you just aren't sure what you can do that could really make a difference. That's where a speech therapist can help you.
For now, just keep talking to him. If you're doing laundry, just talk to him about it. Pick up a shirt and say, "This shirt is dark blue, so I'm going to put it in the darks pile. This other shirt is white, so it goes in the lights pile." When we went grocery shopping, I used to talk to them all the time, "Hmm, should we have pork chops for dinner or chicken? Maybe I should pick up some rice, too." It helps. It really does.
I wouldn't try to make him say things very often. Most of what I did was food related. Asking him if he wanted more (while also doing the sign for more - touching your thumbs with your first two fingers and then touching those fingers of your two hands together). We also asked him if he was all done eating (wave both hands like bye bye and say, "all done"). We also asked him if he wanted "out" or "down." Does he want a drink? Say yes or no. Those were good words to work on.
Only work on things when he's in a good mood. If he's tired or hungry or cranky, that's just not the time to work on words. That's the time to just get him what he needs.
Little by little, you will see progress. But don't worry about whether or not he's a slow learner. There are just so many factors that affect speech. The important thing is to stay positive, keep working with him and see a speech therapist for evaluation and extra ideas.