Him bringing you a diaper when he needs to go means that he IS potty trained... he just isn't going in the toilet. The important part to teach him is to "hold it" until he has an appropriate place to go, and to only go in appropriate places. It sounds like he fully understands than it is not okay to go potty on the floor, and also understands that it is okay to go in a diaper. I'd back off a bit. Leave him naked through the day, or let him pick out some character undies to wear during the day. When he brings you a diaper, ask him gently if he wants to go potty in the toilet instead. If he says no, then put the diaper on him, without seeming frustrated, and let him do his business. Praise him for not going on the floor or in his undies, and then take the diaper back off him. If he allow him to do this in a way he is comfortable, after a while he may decide to go on the potty instead. As long as you are pushing for it though, he will resist. He is obviously freaked out about the potty for some reason and shouldn't be forced.
You could try going to Ikea and letting him pick out a potty chair ($3.99) in whatever color he wants. This will give him more control over the situation. Let him use the chair as a seat or a potty, however he sees fit... let it just be his. Also, when he does go poop in the diaper, take him to the bathroom with you, and dump it from the diaper into the toilet and let him flush it down. This will both show him that poo goes in the potty and how fun it is to flush it down. Once he comfortable with that step, it won't be so scary to go in the potty instead of his diaper. Letting him stand to pee in the potty and aim for fruit loops or something may make it more fun. We bought the Peter Potty urinal online and that made a huge difference for our son because he could see himself peeing.