Cryptorchid is the medical term for one testicle in the abdomen. Yes there is a hereditary factor in cryptorchidism. Although both not descending is more unusual. Is he absolutely male? Or maybe something inbetween male and female instead?
I have some questions with a buck here also, 2nd out of the same doe (different buck) but neither where/are terribly interested in does (although for the second it is also winter and there is some i should do this with doe in my pen behaviour). The doe had a total of 19 kits, 5 bucks, 14 does. His nest sister is fine fertility and mothering wise, but her litter is all does. Too much female hormones maybe? Hard to see without bloodtesting and such.
I found more recently that normal buck - doe balance for wild rabbits is 100 - 130 at best. Don't know what that might do to testosterone production in the bucks.
