I think maybe you just did things in the wrong order.... I potty trained my son at 28 months and here's the order we did it in:
1. Sit on potty and get used to sitting on the potty. reward with sticker whether he pees or not.
2. Allowed to have a diaper free day IF he sits on the potty every 45 minutes. If he doesn't want to sit on the potty he can go back to wearing his diaper. It's that simple... boys who want to wear no diaper have to sit on the potty. Continue to reward with stickers every time he sits. If stickers lose their allure, move to something else like a single m&m or whatever motivates him. We had a chart and after 10 stickers he got a hot wheels car, after the chart was full (4 rows of 10) he got big boy underpants.
3. Wear underpants in the house and remind him to sit on the potty every 45 minutes (if he doesn't pee, remind every 10 minutes until he does). Continue rewarding with whatever.
4. Once he starts asking to go without any prompting you know he's ready to take them in more distracting situations like outside or when friends are visiting. It's too hard for kids to remember they have to pee when they're having lots of fun at first.
So.... keep going but maybe change your strategy. Always pack extra pants and undies. If he resists sitting on the potty put him back in diapers until he's ready to be a big boy. No pressure, but let him know that it's worth his trouble to learn. I wouldn't get out the underpants until he's hitting more than 50% on the toilet if I were you.
My son's been trained since September and almost never has an accident, but every once in a while he resists going pee "just in case" before we go out. A simple reminder that he can always wear a diaper if he's not ready to act like a big boy (eek!) is enough to motivate him now!