Hi XXXXXXXXX~
Small breed dogs (Chihuahuas in particular) seem to have more problems with their patellas than their larger counterparts. Whether it's due to an injury or genetics is absolutely something you'd want to discuss with an orthopedic specialist.
If you'll follow this link: http://www.vetlocator.com/advancedsearch.php , you can enter your city and zip code, and it will pull up all the orthopedic specialists in your required areas.
I think if this were my dog and I was planning on breeding him, I'd want to make sure that the patella problem is due to an old injury...rather than a genetic flaw because, as I'm sure you already know, a genetic malformation like this could be passed on to his litters and I'd hate for your breeding program to have such a black mark on it.
I hope this helps.
Certified Veterinary Technician
Associates Degree in Veterinary Technology. State and Nationally certified.