Using public bathrooms

My 3 year old refuses to use the public bathroom. I have tried everything. Having him pick a portable potty, bribes, etc.
Even at his preschool when all the kids in his class (except 3 who are still in diapers) go to the bathroom he tells the teacher he will go at home. He's still not 100% trained, but when he has no pants on or in underwear only he'll use his potty seat at home.
When I sent him to school in underwear he had 2 accidents and neither time told the teacher that he was wet.
He'll say he has to go when we're out, but then refuses to go when we get in the stall. You'd think he'd at least be ok at school since they have small toilets and all his friends are going as well.
He is very stubborn.
I don't want to carry the portable potty in the car when out bc some times it's not an option to run to the car. Plus, this weather I do not want to sit outside and watch him go in the trunk. Bringing his potty seat is not an option since it's big and I don't want to carry it around. Like I said... I did have him pick his own portable seat out and refused to sit on it.
He still wakes wet sometimes in his pull ups. He also has #2 accidents once in a while too at home.
I have to add when we were at friend's house who has the built in potty seat in their toilet he also refused to go. Only at home he says.
Any advise? Believe me I am trying everything.

Re: Using public bathrooms

Does he have a potty seat at home? Maybe you can get the same exact one to use while you are out.

ETA: This article may be helpful

http://babyparenting.about.com/od/pottytraining/tp/pottyinpublic.htm

Message edited 1/8/2015 2:03:06 PM.

Re: Using public bathrooms

Also, have you asked him WHY he doesn't like using the potty out of the house? Is he embarrassed? Scared?

Re: Using public bathrooms