Stepmom needs help with potting training
20 months is early, especially for boys. Personally, I would let it go and change diapers in the little time that I had with him. Another idea is to broach the subject with her directly: "in the interest of keeping his routine consistent and predictably, we were wondering if you could fill us in on your plans for potty training so we can follow through when he's with us."
A lot of kids become interested in the potty around 18 months or so, then back off and aren't interested again for many months. I wouldn't push potty training, but if you are interested in encouraging him, when you take his diaper off you can ask if he wants to try the potty. If he says no, just say "okay" and put a new diaper on him. And if he does, just let him go check it our, maybe sit on it if he feels like it, maybe or maybe not pee... and as soon as he doesn't want to (even if he stops outside the bathroom door or is just about to sit on the potty) just say "okay" and put a diaper back on. Maybe if you are laid back and show him it is up to him, he will be less resistant because he doesn't think he'll be forced to do something he doesn't want to do.