"...I found out that he did it once before for 6 months and returned on his own to eating."
It's normal for some sexually mature males to go off feed for about 6 months or so during breeding season. 15 months is a long time but not necessarily dangerously unhealthy for a healthy snake. Maybe, maybe not. It's definitely not normal for them to go that long without eating. If it looks real thin & sickly, the it could be at, or near, the point of force-feeding.
It's good that it's under a Vet's care, or, at least that a Vet is seeing it fairly regularly. I hope it's a qualified herp Vet.
Whatever it was eating on a regular basis before it stopped eating, I would try a live one that's slightly smaller. I don't advocate the feeding of live prey but this isn't an ordinary circumstance. Let's say that it was eating large frozen/thawed rats before going off feed. I would try a live medium rat. You'd have to keep a close eye on it to make sure the snake doesn't go to sleep & the rat chews on it. Two, maybe three, hours is all I'd leave the rat in for. I'd also try feeding it during the times it's normally active.
For finicky eaters, offering unusual prey or prey that's too large or too small, rarely gets them to eat. Some are finicky about the size, some about the scent, some about the temperature of the prey, and some that are flat out finicky about all of those. However, stranger things have happened and it sounds like now's the time to pull out all the stops.
If you wanted to try F/T again before trying live, I'd suggest using a medium - large rat, thaw it, wash it with hot water and a tiny bit of mild soap (like plain ol' Ivory white bar soap). Heat the rat under a light bulb or something to between 90 - 100 degrees, to where it's slightly warm to your touch. Then use grabbers or something to move the rat around the cage to simulate that it's alive. Just don't put it right in the snake's face or intimidate the snake with it. Tease it by moving it slightly away from it when it's checking it out. Encourage a chase, but don't make it hard for him to catch/get it.
I know, I know. Washin' a dead rat? I don't know the condition of what you've tried feeding the snake. I was given some frozen rats by a friend that stunk to high Heaven of rat urine & feces when thawed. Even my garbage disposal Burmese wouldn't touch 'em. I washed 'em with hot water & Ivory soap and none of the snakes turned 'em down. I've even done it with frozen mice from a pet store. All I know is - it's worked for me.
More often than not it's a change in environmental conditions that causes changes in behavior. Was the cage relocated or any changes made to heating, humidity, etc, just before it stopped eating? Was a cage mate removed, or anything else that could've disrupted its life?
Any change in temprement?
If at least one side of the cage is glass (see-through) it might help to cover it with a sheet, blanket, or something to give it some privacy during feeding. Of course, you'll have to figure out a way to peek in & check on it every so often, without the snake knowing, to make sure the rat isn't harming the snake.
Is it normally fed in its cage or a different area? If it's used to eating in its cage and doesn't eat the live, smaller animal within its cage, you might consider trying to feed it in another, bare, enclosure. Exercise extreme caution if you do so, because the snake could still be in feeding mode when you go to try to put it back in its cage. If normally fed in an enclosure other than its cage, try feeding it in its cage instead (If it doesn't eat the live, smaller animal.).
Hang in there & best wishes!
HH
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Due to political correctness run amuck,
this ol' hillbilly is now referred to as an:
Appalachian American

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