Your son needs to learn the "timing." To do this, he needs YOUR help. It takes them time to learn how to read the body well enough to get to the potty in time. Your role, then, is to help him learn this, so that he can then tell you. It took my daughter some time to learn this (she fully trained at 23 months, including night, taking herself to the potty and only coming to me for help after the fact. I spent 4 months doing potty learning with her though).
If I was you, I'd only let him run around naked in a certain part of the house, when you have the time and energy to "rush" him to the potty as soon as he starts to pee, or as soon as you see the "poop" face. Then tell him, "pee goes in the potty." After you've had a few "hits," he will start to get it. I'd also put him on a schedule and just take him when you go. Teach him the rules of of toileting, "you go first thing in the morning, before leaving the house, before bed." When I did the "naked" thing with my daughter, I had the potty in the room with us. It helped to get it in her head that when she went pee, she has to go to the potty. Also, always put his poop in the toilet with him watching, and say "poop goes in the potty." If you keep saying it enough, it starts to really sink in, and then they start to head to the bathroom. For what it's worth, I only spent 10-15 minutes a day with my daughter naked. It took us months, but she was young and I wanted to introduce the concepts she needed slowly and at her pace.
Whatever you do, don't turn back. Hang in there, show no negative emotion, and be patient. This is a giant developmental milestone and they need our help in teaching them. One thing that helped me get my daughter to go poop on the potty was to talk to her about what she was feeling. So, when you see him going, ask him if he feels the pressure, and tell him that when he starts to feel pressure, it's time to go try to go to the bathroom. Talk to him about it too when he has gas and ask if he'd like to try to go on the potty.
There is a window between 18-24 months, so good for you for noticing his interest! It is so worth the effort and time!