Elderly cat and incontinance

I have a 17 year old cat. He has an overactive thyroid which he is on medication for.
In the last few months he has been having a few bum related issues. He is an outdoor cat and still goes outside to go to the loo but sometimes when he is relaxed e.g lying on the sofa, a bit of poo will squeeze out. I have been to the vets about this and they say due to muscle deteriarion, he probably isn't managing to 'squeeze out' all the poo as normal so when his muscles relax, what's left comes out... Has anyone had this issue before? We haven't had any full blown accidents... Just dribbles. He doesn't have diorreha either. I'm clueless. I don't know whether this is something we will just have to deal with or something I can help fix. Atm I'm just wiping his bum when I notice bits coming out to make sure he is clean and preventing too much of a mess

We had this problem with an almost 19 year old cat we had.  When he got older, he just couldn't control himself as well.  Poor guy tried to get to the litter box but couldn't make it in time.  We ended up just making sure there were plenty of puppy pads where he slept and around the litter box.  

I'd have him checked by the vet to make sure there isn't something new going on medically but it could be just age-related.

Thanks for the reply. The strange thing is, he doesn't even notice it. It happens when he is asleep and relaxed, and it isn't much, just like small drops every now and then. He still manages to get outside to do a full on poo.
He has regular vet trips anyway regarding his thyroid issues and it was the vet who said it is probably loss of muscle in that area but just wondered if there was anything I can do. I hate worrying about him