I'm guessing it is a bit of arthritis which is basically stiffening joints this will have been exacerbated by no turnout. Anything stiff is better out 24.7 - proved by all those with 30 yr olds loving living out and barely stiff at all.
And just to show what his feet looks like inside (very clean and dry so I am positive it is not thrush making him sore):
Took him for a little hack today and he was on his toes and didn't feel stiff at all so I don't think I need to be too worried. Will still get the vet to take a look next week though to be on the safe side.