Son is refusing to potty train and we are getting overwhelmed

My son just turned 5 and will not potty train. We've been trying for a couple of years. He's adopted and we knew he suffered from abuse before coming to our home. His mom was physically abusive and his grandfather was sexually abusing him at 2. We've showered him with love. We don't spank him. We try to teach him to express his feelings. Potty training has been a nightmare. It's why we haven't sent him to school yet. He still goes to daycare but he will have to go to school next year. He won't even sit on the potty. He will start screaming. People judge us harshly in public when he's melting down and when people find out he's not potty trained they look at us like failures. My husband and have no idea what to do anymore.

Edit: He's not in therapy and he hasn't been evaluated for autism. I'm definitely going to look into therapy. We've both tried really hard to give him the love he deserves in hopes it will help heal that trauma but it looks like it's not enough and we need outside help