
In my breed I would not count a slip mating at all, but I know that slip matings are common in toy breeds where often the Male is smaller than the bitch, but having had it happen once I would have expected the stud owner to hold the pair in close contact.
Ties are more satisfactory as you know for sure the goods have been delivered, rather than with a slip it's like the paperboy riding past on his bike and throwing the paper at the door (as in American films).
Also fertility in males starts to decline from 5 years onwards, not maybe enough to be much of a worry in natural matings with plenty of opportunity for delivery of useful amount of sperm, but does affect the viability for freezing for AI.
At 10 years old in my breed where males get little stud use I would be thinking myself lucky to get a litter, (though of course it's entirely possible), has this male recently produced puppies?