So my ex-gf has my 5yo son living with her, and she stopped me from seeing him for about 12 months until the court ordered that I should see him every weekend at least.

Since the time she cut me off from him, he has had separation anxiety issues. This has been identified by a qualified psychologist.

How would his separation anxiety be resolved?

My first thought was simply bringing him back to regularly allowing me to see him again, but then thought that it would only “band-aid” the issue, as it would still potentially leave him with concerns that it would happen again.

Honestly, these answers would be better answered by a trained psychologist than by anyone here.

Talk to his psychologist. S/he’s in a far better position to help you than random, unqualified strangers on the internet.

JTQ said:

So my ex-gf has my 5yo son living with her, and she stopped me from seeing him for about 12 months until the court ordered that I should see him every weekend at least.

Since the time she cut me off from him, he has had separation anxiety issues. This has been identified by a qualified psychologist.

How would his separation anxiety be resolved?

My first thought was simply bringing him back to regularly allowing me to see him again, but then thought that it would only “band-aid” the issue, as it would still potentially leave him with concerns that it would happen again.

Kids trust you as far as you throw yourself. As long as you are their and reassuring them their fears are reasonable but not forgotten they will just tag along with your best efforts.