Update guys:
No change to him.
I mean, apart from being on his side (or upside down), wedged into a piece of wood, he seems completely fine.
He eats well, he reacts, he adjust his position every now and again...and always chooses to go back to the same position in the tank. He still looks amazing, nothing visibly wrong with him at all.
It's weird, I guess this is just how he is now. It's difficult, not have a lot of information about the species. If there was something wrong, I would have expected it to get worse by now, it's been like 7 months or whatever. Maybe, as they get older and bigger, this is just how they go sometimes....He's probably like 6 or years old.
....
I introduced the peas. Cut feeding a little, and increased the amount of water changes just to see as well.
I also added a few platies to check if there was some contaminant or something in the water that the catfish had somehow got used too...but they're all fine, breeding away as Platies do.
....