Hey, R. ...
In my case, I just had to talk the kids through (no tin these words) 'sometimes ideals have to bow to reality' ... teaching the child that until they could make it to the potty consistently, they did still need to wear (the cloth) diapers, but keeping their diaper dry was great ... and there were times even after we were in underpants, when I would put them in diapers situationally (visiting somewhere so interesting that the likelihood they wouldn't notice they needed to pee was high, long cartrips, when I knew they were going to go down for a hard nap, and ESPECIALLY when they were sick ... I STILL diaper the older ones if they are really sick). The "keeping the pee in" ideal is still the goal, but the "Mommy can't change reality" had to be admitted.
He can probably learn the "at this house we wear diapers" idea. You'll have to suss out how to deliver the message so he doesn't hear "I don't expect you to use the potty at this house" ;).
I don't know that I'd recommend it, but you could consider an "if you are still wearing the same diaper, dry, when we get home" acknowledgement, and I had some success with telling the kids early "as soon as you go (x) days without an accident, we'll try underpants (in that situation)" ... this might require a sticker calendar or something, especially for someone that little ...
I missed the window of opportunity that you have caught with my first, who really was ready at 18 months, and 6 months later tried the sticker-rewards method ... #2 and #3 I caught when they were ready (age 3+ and age 2+, respectively), let me tell you what! I think it's great that you are listening so closely to your son.