My son is 20 months old and I have started potty training him. He has been interested in the toilet for a month or two. He will pee and poop whenever I sit him on there, but still won't let me know when he goes when he isn't on the toilet. I let him either run naked or put his big boy underwear on, but he stil won't let me know when he's soiled himself. What do I do to help him tell me? He will say "pee pee" after he goes in the toilet so I know he knows he's going...
I think you should wait a bit. Ideally, your child should come to see you right after soiling his diaper to get it changed.
We tried very hard to potty-train my second daughter, but it only worked when she would come see us to get a diaper when she was naked and needed to pee. Before that moment, she wasn't ready. That was at about 30 months old, by the way.
Twenty months is quite young to expect a child to be potty trained. It will happen when he's physiologically and developmentally ready, and there's not a lot you can do to move it forward, and good reasons not to try: frustration and icky messes on your part, tears and shame on his part, and a lot of conflict between you two to no purpose. If it isn't working, he's just not ready.
It seems a bit early for me, too although my daughter was potty drained at around 27 months (she decided that, not us, even though we also tried earlier).
I would suggest to make a routine out of it: Before or after sleeping, meals or similar. I would also keep the diaper on for the time being. The benefit for you may be that it may save you from having to clean him up because he regularly does it on the toilet. That's good for you without too much expectations.
At some point, he might tell you in advance, or he might wiggle and you'll realize he needs to go, take him then and see if it worked.