Hello and welcome to Monty. Beautiful Maincoon. We are seeing a few of these lately and both with hind limb issues.
This one has had mobility troubles in the hind since he was less than a year old. Having seen the specialists he was diagnosed with bilateral hypdysplasia and luxating patella. Both are not too severe but all together add up to some discomfort.
He is keen and happy and still likes to climb, though jumping is difficult. So we can manage this with some muscle building.
So we have set him up with a home excercise regime. Yes, we can get cats excercising!
Good luck dad, keep up the good work. We'll have you feeling better soon Monty!