The first thing you do is contact someone who shows Chihuahuas or a competent vet. They can tell you if your pup has the proper proportions, and is not in showing signs of common skeletal issues known to the breed. They would need to check to make sure his knees are healthy and not likely to get Patella luxation, which basically means loose kneecaps also known as slipped stifles. When this occurs the kneecap of the rear legs slip out of the grooves and can happen repeatedly depending on the severity of the problem.
They also need to check to make sure his mouth is not too small. This can lead to teeth crowding which can lead to tooth loss from food getting trapped. (If you are not brushing your pup's teeth, you need to start.)
You should also have him tested to make sure he is not prone to hypoglycemia and hydrocephalus. Both are common in the breed.
Once you have a clean bill of health, you ask you vet or show person if they know of a female that is also healthy and clear of defects whose owner is willing to breed.
If you pup isn't cleared as a healthy pup, get him fixed. You do not want to breed knowing you could be breeding a pup that is going to be in pain later.