DS is recently three and has had access to the potty for over a year now. In the past couple months we've gotten more consistent about trying to use the potty every time he has to pee/poop. He's now in underpants all day until bedtime. The thing that's frustrating is that he only seems comfortable going potty at home. He did use a restaurant toilet (with DH's help) back in February, but otherwise, he won't entertain the idea of going potty elsewhere. My parents recently bought a little on-the-toilet potty seat for their house, but DS doesn't want to use it. Additionally, he has said he won't go potty with anyone other than me or DH. We've been working on this since one or both of my parents is at our house babysitting at least once a week. I think there have been two times (one recently) that he actually used his potty while DH and I were not here and his grandparents were here. Other times he just pees in his underpants when they're here.
This obviously makes staying in underpants tricky if we're going to go out n' about. He'll be starting preschool (3 mornings a week) next month. He's not expected to be toilet trained by then, but it seems silly to send him to school in diapers when he's been wearing underpants during the day for a few months.
Any ideas about how to help him be more comfortable with expanding his potty experience range?