I teach early elementary at an alternative school similar to the one you are describing. My goal is to work my way out of a job, so to speak, by successfully transitioning my kiddos back to their LRE as they reach their behavior goals. I've had plenty fully transition back within 6 months, but I've also had some for several years. There is no denying that students see other inappropriate behaviors. Part of my job is to be continuously modeling for my students the appropriate behaviors and to be providing plenty of positive feedback. Hopefully this is what is happening at your son's alternative school.
That being said, most of my students have transitioned back to their school (I only have 1 student now who has been with me for more than 3 years). This is done with a LOT of support from across the board. First, meeting with the parent monthly for a formal review of the student's progress. I stay in contact with parents more frequently than that, however, with phone calls every week or so and occasional home visits to brag on their kiddo (students LOVE it!) While you cannot get your teacher out to the house, requesting a meeting to review your son's progress, or, if applicable, struggles, is a great way to ensure you are in the loop and have documentation of your son's behavior and the school's interventions.
Support staff is incredibly helpful during the transition process. In my opinion, it is most helpful when that person builds a rapport with the student at the alternative school prior to the transition beginning. Furthermore, I also encourage my students' teachers to visit our school periodically to maintain their rapport with the student - returning to a larger environment will be stressful and you should expect bumps in the road when that time comes, but the fewer changes the better and a few familiar faces when he starts to transition can only facilitate the process.
The alternative teacher is likely teaching some replacement behaviors. During the planning, ask your son's teacher at his regular school how those replacement behaviors and coping techniques will be implemented there (For example, if he has a cool off area, where will that be in his new classroom? Who will be responsible for practicing with your son the procedure for asking to cool off? Does he have Occupational Therapist (OT) aides, such as stress balls or vests? Where will those be kept and what will be the procedure for your son having access to them?)
Each student I've had has been different and we've developed the transition plan to best address his or her needs. And each school district is different. If I could only offer one bit of advice in preparing for, and during, the transition process, it would be to maintain communication with the teachers. If they don't initiate it, you initiate it and request a meeting. It does not need to be an IEP, just a meeting to make sure you are on the same page.