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?