My Son Won't Potty Train

We've been trying for years. My son is 6 now and isn't potty trained. He is for pooping, but peeing for some reason he's not able to get to the potty. He's getting really upset and frustrated because he really wants to wear big boy underwear. He wets the bed, which is pretty normal for a 6 year old boy, but peeing himself during the day. I just don't understand what we are doing wrong. I don't want to make him feel bad because he starts crying every time he pees his pants. I can tell he's trying because he's going poo in the potty. His frustration just seems to be getting worse. We've tried everything. Idk what to do now.