Thank you very much for the replies.
I got him to a snake breeder friend who examined him today & agreed with you both - it's just a long scratch, no scale rot!
In terms of feeding though, he's definitely way too skinny & was also a bit dehydrated. My friend got him to feed with a particular rat that ball pythons apparently really enjoy. Now here's hoping he won't regurg them again.....
Previous to this, he refused to feed for 6 months & then finally ate in the fall, which is why I was trying to feed him through the winter. Weirdly enough, he didn't loose any weight during the 6 months, but then got too skinny in the past month.
Thanks again, I'm so glad it's not as bad as I thought.