
Slipping Patellas are common in small breeds of dog as the groove the kneecap fits in is too shallow. Very common in the ubiquitous Jack Russell types, so much so that many people think it is normal for them to have the bunny hopping hind gait.
There is no official screening scheme for this condition in the UK, though in the USA you can get certificates of freedom from this condition through OFA (Orthopaedic Foundation for Animals) after an examination by an Orthopaedic Vet, and people in many breeds avail themselves of this, usually when having Hips x-rayed for Hip Dysplasia.
I don't know how much of a problem across the breed this is, so it could be just bad luck that yours has this problem due to being a small dog rather than any pattern of this problem in his breeding line.
You should inform his breeder so they are aware and can adjust their breeding program accordingly, a breeder can only plan based on what they know.