Talking to you family members about how you would like them to be toward you is not the way to set boundaries. I mean it might seem like a start, but actions that enforce your boundaries is what gets the job done. When they start haranging you about how he shouldn't do this or that, quickly change the subject. If you hang on their every word when they are critical, that encourages them to talk that way. Some of what they do is attention seeking. Don't reinforce bad behavior by paying a lot of attention to it. Act bored. Refuse to discuss how they think he needs to change who he is.

You are trying, and you are learning. You can get even better at curtailing their inappropriate conversation. Right now you need geographic distance from them. Eventually, you'll learn to keep a mental space between you and them, even when you're in the same room with them, sitting at the same table.

Of course he felt ganged up on. When they are guests in his and your home, they have the obligation to act as respectful guests.