Well done on leaving him. I have been in a similar position and i know it is very hard to do.
Your son will have been affected by his alcohol use over the years however much you tried to protect him. You don’t say in your op how old your other children are, but if they are younger it might be they have been shielded from a lot of it.
Your son is possibly affected by all the change, with Covid and lockdowns, so much have changed. It will take time.
The most important thing is he has a happy, stable and loving family home with you, and I am sure you provide that. You are strong- don’t forget that.
Have you thought of refusing contact and let him go through the courts? Cafcas would then do a report, and contact would be ordered. Irregular unplanned contact is not in your sons best interest.
All your children need to feel safe and secure, having routine is key to this. Unfortunately, someone who is addicted to alcohol (even those described as “high functioning”) do not seem to understand the importance of this.
I don’t advocate parents refusing contact, but when it is at the best interests of the child you sometimes need to look at this Avenue. Courts, for example, could insist on contact being at a contact centre for a specific duration, they are there to put the child’s needs first rather than the parents. This would mean the children are safe when he has contact (I am assuming he takes them for a few hours).