I would think that sperm count could go down in a male with age, I usually base breeding capability on overall health, not age, in reptiles, because one 10 year old with great care can be healthier then a 3 year old with moderate care.
I also wanted to ask you, if he was a rescue, and therefor you don't know his genetics, why would you breed him? In my opinion, this would be the reptile equivalent of a back yard breeder in the dog world. This is not meant to attack you, but hopefully give you something to think about.
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