I have had several stallions, Haflinger, AQHA, Hackney(pony), Welsh, & Mini. never did much different than with the mares? Did have 1 which was nervy about the crupper so just left it on while in the tie stall , kept it loose at 1st then shortened bit by bit. Usually had them going long before 4th birthday tho?? The AQHA stallion was 7 before being ridden 1st time tho and it was easy as pie.(mostly)

I've been an AQHA breeder for over 20 years, have stood several stallions, can think of no reason a stallion would have more issues than mares or geldings in this regard.

But would give serious thought to WHY you need the crupper: if the saddle is properly fitted, it should stay in place without this aid.

most harness types have a crupper, especially driving types. I usually had mine in harness before under a saddle.

Thank you both for your responses. The reason he is four before starting him, is due to having a broken leg one year and needing it to heal, this year I was uninsured for a few months, and after breaking a leg, I am not going to risk breaking a horse. The reason for the crupper is simple, its hard to drive a carriage without a crupper. Yesterday I saddled without the crupper, then used the crupper. He did really good. Today, there was no buck at all. Actually, he did quite well. Thanks again.

How soft is the crupper?

You may have to wrap it in something soft so it doesn't bother him. I have one horse who is VERY easily upset by anything touching his tail. The mares don't care, as long as it doesn't pinch.

The crupper may not be the correct thickness for this horse. Might be too tight or just rough under his tail bone. I would start with cleaning the crupper, make sure the leather is totally smooth, flexible, soft to the touch. Check under his tail, make sure there is not dirt underneath bone to rub on crupper. No sores at the edges of tail bone from too narrow a crupper rubbing or perhaps too tightly strapped in from being too small. Some horses have a wider or thicker tail bone, need a bigger curve in the crupper. Be sure to pull all tail hairs up and out of crupper. Don't leave any hairs under crupper to rub the smooth underside of tail bone, it will sore him up.

I also would advise starting with loose crupper, then gradually tighten backstraps up as his tail relaxes during work. This could take a couple weeks, no hurry. You want him accepting, not crabby. Eventually you will have crupper correctly adjusted when training, plus a comfortable horse.

You might just pick up another crupper from harness shop, try a different model to see if he likes it better. Cruppers are pretty inexpensive. Do check for leather smoothness, no flaws or scars on hide because they will rub him sore. You want a large gentle curve shaping to go around tailbone. Not too thick or overfilled, not thin either. No tiny curves to pinch him.

If the weather was cooler, I would also suggest warming the crupper before harnessing him. We have had some object STRENUOUSLY to cold cruppers. It is not pretty to watch large horses trying to scoot with cold harness! And the UGLY LOOKS after!! Much easier to warm the crupper up FIRST!