Thanks for your replies! A baboons bum, now that made me giggle!
Yes, I would say it looks very much like scar tissue and it`s dry, not weapy or red. What I`ll do is, watch things closely and if it stays the same, I`ll know that`s what it is. If it starts to spread upwards or he loses more fur, he`ll be taken in to my vet to discuss probably causes.
Both my roborovskis are lovely lads and came from babyboos on here last year and they are thriving. x