@Joziegirl - it’s either a training problem, or he is not the right horse for you. You decide.
Altering the temperament of horses in order to show is unethical and forbidden. It is not in the horse’s best interest. Whether Regumate works on geldings or not, it’s not approved for use in males because they lack the the biology that the drug was created for.
To ask a question on a public forum, and then tell people what answers you will accept, is a “ya, but”. You are not asking for advice, you are asking for affirmation that what you are trying to do is ok. It’s not. Most members of this forum won’t tell you it is. Since you are a “greenie” on this forum, I am guessing this name is an alter, and you already know the pile-on you will receive. The answer you seek, grasshopper, will probably not be found in this forum.
Why would you ask your vet’s advice, then go to a forum for the advice of a bunch of strangers? I think because your vet has told you that it’s a bad idea, unless you use Dr. Feelgood, and then you wouldn’t need this forum’s advice, because Dr. Feelgood would have just given you the drug and shown you how to use it.
Finally, I suspect that you will go to other locations looking for the answers you want, please know that Depo is “BANNED” not band.