First things first...don't push it. The more of an issue you make it, the more likely he will balk. My son was doing great with his potty when he was nearly 2. His grandmother wanted to "help" despite my pleas that she not. She is a very pushy and insistant person...My son refused to go near a potty until he was 3 because he was 'traumatized' We had to start from scratch.
Home is his comfort zone. You might start moving the potty to a different location; so he knows that there can be more than one location for a potty. Then have him practice at home with his clothing on, so he gets used to having to disrobe before usnig the potty.
Then go on to using the potty in the public; but remember, he has to become comfortable with the idea of doing it in a different place. It can be scary when it is new.
If he pees in his trainers after he was prompted to use the potty in public, don't get too wound up about it (I know it is really frustrating sometime...and kids pick that up even when we don't think we are obvious). Just go, "oh, that is a bummer you had an accident honey, but that is ok. Maybe next time you will try the potty and you won't get wet." Knowing that your current opionion of him doesn't hinge on using the potty at that moment will make it less of a big deal; which will increase his comfort level.