My thought is very simple. The school is saying he is emotionally resilient and will have little issue with changing schools. Where's there data on this?
According to the way rules are written, schools can generally do things like this. There is nothing that says they need to consider a student's resilience or mental health status when making this sort of decision, so they are in their right to transfer him because there is nothing that says anything to the contrary.
I see this as the ball being in your court and you needing to strategize how you will return it back to them. If you are concerned that he will have 2-hour long meltdowns if he has to attend the school they've picked, mention that you have concerns about school refusal and request that the school evaluate that this would be OK (that he can handle this without tantrums and other disruptions to your household) given that you see his emotional dysregulation/disability being a big factor with making a huge change like this.
Do this in writing so you have it documented in a paper trail. Write this out as a parent concerns letter. (More info here on writing a letter: https://adayinourshoes.com/parent-concerns-on-the-iep-parent-letter-of-attachment/.) Request support to help him make this transition. I know that some of the things that can help are being able to explore the building over the summer or in off hours, meeting with staff. Playdates with other students (this would depend on the other student's families being OK with this). Also request an evaluation that this isn't going to trigger anxiety and school refusal. My perspective is they decided to make this change and you need to move forward from where you are which is him attending a new school where he's unfamiliar with the floor plan, staff and classmates. You need to plan your strategy from here. My thought is that if he doesn't cooperate with touring the new school & meeting his new teachers (things that are in the IEP because you requested the school support him with this transition), you now have data that shows this is causing him anxiety which is something they didn't consider when changing his placement.