IMO, read up on coparenting and parallel parenting. Keep things businesslike, as much as possible. It's about the kid, not each other. Family counseling is a good idea. Have your son visit the school and see if there are any groups or activities that he could join in the summer to meet new kids from his school.
In my experience, anybody who wasn't stepping up and then suddenly does to the Nth degree will burn out.
If you have not already done so, tell the school and teachers of the new arrangement. My sks' teachers knew exactly what Mondays followed their mother's weekends, and what projects where done on her time (we didn't tell them specifically, they just figured it out). Ultimately it is between the teacher and the kid what gets done and what grade they get. My SS was always better about being self-motivated than SD was. We made sure their mom had the same newsletters and forms that pertained to her that we did, but she had access to the same school calendar we did, so we didn't remind her about 3 day weekends unless she seemed to have forgotten.
I would also expect the divorce itself to affect schoolwork and school behavior and would give him time to adjust.
Also, whatever you decide, stick with it a while. Kids need to know "Monday through Weds, I'm with Mom and Thurs through Sunday I'm with Dad". Everyone wants to see their kid every day, but sometimes it can be worse to do that vs week on/week off or the NCP having every other weekend and holidays.
ETA: My sks' mom works retail, too, so holidays were especially hectic. If you don't yet have a court order about the schedule, please try to get one. The kid needs to know what house he's going to after school, if he can leave his lunchbox there overnight, etc. I realize that your ex is not in total control, but many people can say "I always need Wednesdays because that's the night I'm scheduled to keep my son overnight."
Also, re: differences, unless the child is in danger, I'd let a lot ride. My sks knew what they could get away with in both houses, just like they knew what would or would not fly between home and school. It often took a day or two to readjust (another reason for a schedule) but the kids remembered that we don't eat in front of the tv soon enough.
It'll be OK. Keep the long view.