couple of notes:
I hope your fence isn't one made of treated lumber. Older treated lumber has arsenic in it, and could be deadly if he ingests it.
My dad, in all his wisdom, (now gone to a better place) said
years ago, when he raised beagles for hunting, he would spray bleach around the outside of his pin when he had a bitch in heat. Said it deterred the males from trying to gain entry.
Does he do this when you are watching, or does he do it when he is alone? if he does it while you are around, a shock collar might work. Doesn't hurt them, but never seen a dog yet that liked it. A simple association of "I did, I get zapped" kinda thing.
The hip problems that a lot of breeds have issue with, esp larger breeds, can be checked out by your vet with simple x-rays. Also will add to the amount you can charge for stud. One of the reasons some breeders are so proud of the time their male will spend with your female!
The other option is to concrete your whole dog area and paint it green! LOL