Its not unusual for those who take over barns to write up new boarding contracts. The only difference between you and those others is that they usually do it immediately, not when there is a problem.
I would spend an hour to two and write up new board contracts for every single boarder in your barn. If they don't like the "new" rules, they can leave. Have in the contract that you will provide dewormer unless otherwise discussed (and the cost will be added to the bill), and if the horse is in need of farrier or vet work and the owner can not be reached, the BO has the right to make a decision based on the horse's welfair, and the cost will be added onto the board bill.
All horses will be kept at the best health possible or else evicted to keep all others healthy. You may even want to consider asking every boarder to obtain copies of vet records and a cogins from their vet for your records. Most barns won't let a horse into their facility without updated shots and a negative cogins. Now that you own the barn but don't know squat about records, you have to start records to prevent this from happening with anybody else.
Make the contracts, get the boarders to sign it. Make sure every single boarder knows NOT TO TAKE THIS MAN'S BOARD CHECK to give to you. Tell them to deny him and tell them that if he tries, they are to direct him to you personally. You need to see him about a new boarding contract He is avoiding you.
Personally, I would deworm the horse if its causing his health to dip. I would also schedule him for a cheap trim. You may get your money back, you may not. But I couldn't let a horse suffer on my property while knowing I can afford dewormer and a cheap trim for him to keep him stable.
It isn't unheard of to own a horse and not see it. I've owned my mare for a year and because of personal issues, haven't been able to see her until the summer months. And even then, I'm lucky to see her when I have my daughter with me (shared parenting). I expect my BO to make sure my horse is taken care of, because that's what I pay her for. That's what boarding is about. She doesn't see it as neglect since I don't see her as much as I want to. If there is a problem, we discuss it. But my horse has been properly dewormed and trimmed for months without my presence. That's why I board.
She was thrilled to see that I was finally back in the area. Not because I can take care of my own horse now, but because I can finally enjoy the horse I've been paying her for.
Print out new contracts. Make it so this man has to see you personally in order to pay for his horse. You won't get any answers if you let him avoid you. There are ways of finding people...if you have his first and last name and his general where-abouts, you can pinpoint him rather easily.
And those who were seen dancing were thought to be insane by those who could not hear the music