You like the idea of a man aroused and kept on the plane of sexual excitement for some time. Perhaps you'll let him come after you've had your fun; perhaps you won't. Release is nice, and it's fun to see him completely lose it when he orgasms. On the other hand, you could just make him go away hard, and he'd have to deal with the unwanted erection until it went away, and he'd remember what you did to him every time his pants rubbed against his sensitized prick. And that's a thought that gives you lovely tingles too.
You could even tease him after you sent him away. Make him text you after an hour and tell you if he's still hard. Would the excitement of having to report back to you keep him turned on? Make him tell you in detail every time he thinks someone noticed his hard-on. (He wouldn't, of course, be allowed to try to cover himself up beyond wearing pants and sitting down sometimes. No jackets, no books held in front of him. You want him to feel embarrassed and exposed and like everybody knows.)
You could have someone you know--a young, sexy woman, perhaps, or another man if he's bisexual but feels men are off limits for some reason--go up to him and engage him in conversation when he's in a position where he can't cover himself. Maybe you'd set it up so he's just received or sent an explicit text message, and it's making him stiffen again if his cock was flagging. And then you send in your accomplices. Have them flirt with him, maybe, or just pose provocatively. You know by this point he's thinking of sex at the least excuse, so you could have them behave just within the bounds of plausible deniability. That way he'd feel guilty for turning everything sexual, and the shame would turn him on even more. Or you could have them be more overt. What if they touch his arm or get up in his space or let their eyes travel over his body, lingering at his cock? What if they proposition him?
Of course, if you do ask them to proposition him, there would have to be limits. You don't want him getting off without your explicit instructions. So maybe you'd have told him he wasn't to touch himself. Maybe you'd give him conflicting instructions: no touching his dick, no getting off, but submit to anyone who comes up to him and asks to touch. And then when they jerk his prick, he'd have to decide if that counts as having his dick touched. The solution, of course, is to message you, but if he does that he'll have to explain in detail exactly why he has this problem. That would be delicious for you and humiliating for him.
Maybe instead you'd have him listen to you on the phone while you orgasm. Whether you make him take his cock out--but not touch it, no, you want him to have the frustration of wanting touch and not getting it--or make him leave it trapped inside his pants so the hard-on hurts, he'll get even harder hearing the sounds you make when you come. You could tell him how much it turns you on knowing that he's stiff and throbbing and unable to do anything about it. You could make him describe to you exactly what he's feeling. You could have him photograph his erection and send it to you. And then you'd tell him that you're thinking of how good his cock would feel inside you while you come. He'd have no trouble staying hard for you to play with after hearing that.
Should you make him wear underwear, or not? The advantage of underwear is that it traps his cock and balls tighter. You like the idea of him being restricted and restrained, the thought of the erection that's usually so pleasurable for him being painful as it's pressed tightly under several layers of clothing. You'd want to watch him put his clothes back on just to see how uncomfortable it makes him trying to squeeze that long hard rod back under his briefs. The advantage of having him go commando is that there's only his trousers covering his hard-on. If you did that you'd make sure he was wearing thin, clingy pants, something that showed every line of his cock and balls when he moved. That would be great for when you want him to feel ogled. It would be great for any of your friends you want to let have the treat of seeing his cock without having to undress him, too.
A cock ring would keep him hard for you longer. Keep him nice and stiff with a little less input from you. You might have him take his cock out and stroke it sometimes--of course if he did, he'd have to tell you all about it. Make him edge himself. Maybe make him stroke it for a specific length of time or a certain number of strokes. Or make him jack his dick somewhere with a risk of discovery--under the desk at work or school, perhaps, or when there's someone else in the room, or when he's on the phone and can't make any noises or the other person will realize what he's doing.
If you let him come, it'd have to be something humiliating. Maybe he'll have to come inside his pants. Maybe he'll have to get himself off by rubbing up against something not made for the purpose--a sofa arm, the edge of his desk, the floor. Maybe he'll have to get a hot lunch, put it on the desk in front of him, jerk off over it--and eat it that way.
So many choices, so many ways to keep him hard and at your mercy. Mmm.