Unfortunately, once horses know they can get out, they are much much harder to keep in from then on. I had one that would jump over fences if he couldn't bust or go through them. We tried it all, and ended up with 5 foot no climb with a hot wire topper 6 inches above that, and gates set high with a hot line strung over the top of them. And even then, he still managed to get out. And it didn't matter what sort of pasture he was on, or how many herd mates we had, he still did it. I watched him bust through a fence to leave a 40 acre pasture of grass and his 4 best buddies, to nibble scrub grass and weeds on the edge of a crop field. He passed away at 29 from cancer, and still managed to jump out over the gate not even a week before he passed. He waited on the outside of the gate, never even walked off from it, he just escaped out of habit.
Now that you have an escape artist, you will have to be extra diligent about fence upkeep for as long as you have him.
"Keep a leg on each side and your mind in the middle"