Separation Anxiety in Grade School-Aged Kids?

My son is almost 9-years-old and his dad and I split up when he was 1. Starting out I had about 90% custody and my son didn't have any problems separating from either of us.

As he got older his dad became more present and his share of custody increased and my son started crying at drop offs to dad's house and saying he didn't like daddy. Please note I have never had any inclination of abuse and I know my son loves his dad and has fun once he's there - it's the act of separation that he had a hard time with. The crying at drop offs ended around age 4.

We've now been 50-50 custody (7 days on, 7 days off - which I don't like but my ex got a judge to agree with him) for a few years. In the last year or so my son has been having a hard time with separation anxiety again.

He starts feeling anxious the day before our exchange day and cries when I drop him off at school on exchange day. It's gotten so bad that even when I visit his school for an event, he cries when I leave, even if I'll be back to pick him up in a few hours.

When I ask him what he's worried about he usually says he doesn't know. Sometimes he says he's worried about me or what might happen to me while we're separated. Sometimes he says he just doesn't like being away from me.

He also has a really hard time making decisions sometimes - even if it's something that doesn't really matter. Like I asked if he likes having the flat sheet on his bed or just the comforter - and he got stressed out about having to choose. Not sure if this is related.

Any insight or tips how to handle this at this age? He has a family photo album in his backpack and we tried letting him keep a worry stone in his pocket.