Actually, the term proud cut comes from people colloquailly referring to the epididymis as the "pride" - this is a testosterone producing gland that is wrapped around the testicle and sometimes a supporting ligament. During a standing or prone castration, the vet has to reach up into the body cavity and carefully peel this glandular tissue away and remove it. Just removing the testicles and not the gland, or not taking proper care to peel all the glandular tissue away from the supporting tissue results in "proud cut." Assist in a few castrations and you'll be surprised that there aren't more cases. I think this is way more common than cryptorchidism. Depending on how much tissue is left behind and how much testosterone it produces, your gelding can do a pretty good impression of a stallion.
Most of the OTTBs I've had were cut late and exhibited some stud like behavior, but nothing unmanageable - mostly an unseemly interest in mares and a willingness to court and tease one . I've alway wondered whether it was learned behavior or truly a result of being cut late.
If your horse is managable, I wouldn't worry about it.
I had a gelding that did the same thing, just studdy around mares. He would seperate the mares and geldings in the pasture. He just liked his girls I guess...I never caught him mounting mares, just same as you said arched neck etc...he was a bit of a handful on the ground but I think his was more of lack of training and turnout than proud cut. I think the only way you will know for sure is to get a blood test done on him to see about his testosterone levels, as has already been said, to give you a better idea and to rule out if its that or just lack of training and attitude..how old is he?
I had a gelding that would breed mares in the pasture untill he was almost 20 years old. I never tolerated any expression toward another horse gelding or mare while I was riding or handling him so that was never much of a problem.