Potty Training, but only at home?

Cooligan

My 3-year old son recently decided, all on his own, that he doesn't want to wear diapers anymore. We've gone along with it when we're at home (he still wears one out, and to bed) and he's done remarkably well with only a few accidents. He's clearly very proud of himself and getting more confident by the day. This has been consistent for about 5 weeks now.

The problem is, he absolutely refuses to sit on the potty (even if I bring his over, which I know he's comfortable with) at my moms. Now, he spends all week at my mom's while I'm at work, and he loves her more than anyone... so I know it's not that she makes him uncomfortable or that he doesn't trust her.

 How can I generalize the sudden interest in potty training to other environments? I don't expect him to be comfortable in public yet, or at other homes, but I'm shocked and frustrated by the resistance I encounter at Grandma's house!

Cooligan

We haven't challenged him to go diaper-free while we're out, yet. But, last weekend, we went grocery shopping, and he was mad that we made him put on a diaper. He told us he would not pee in the diaper because he didn't NEED to wear a diaper. We totally forgot that he even said it, but when we got home, 2.5 hours later, the diaper was bone dry. Maybe we'll start with trips, and then try grandma's house again.