[Starr05]

I had a Boston terrier that would do that, we took him to the vet and found out he had a luxating patella(sp) its when the knee slips in and out of place.

 

[Starr05]

If it were me(only because i have been throught it) I would take him to be seen. It does sound like a Luxating patella. It can easily slip in and out of place. If its a grade one usually they will only keep an eye on it and have you give him a joint supplement.