I guess this would really be more "edging" than milking, but I did get to play out a fantasy once - being a "doc" taking a "sperm sample" from a consenting yet sheepish "patient" - we set it up that he needed the sample taken to complete his "sports physical" and the rule was I had to help him produce the specimen. Of course, we played it that "patient" was straight and had never been pumped off by another guy before - but since I was a "doctor" and he really needed this result for his records, he agreed to let me do it.
My stroking his cock seemed awkward for him at first, but banter about sex - especially about girls he liked (hey, guys, it's fantasy, lol) soon got him fully stiff and ready to let go with a sample. That's when I stopped stroking him, to his confusion.
I explained I needed as much sperm as possible for this exam to go well, and that the more he held off, the bigger his load would be once I sensed it was time. Questions about how long this would take fell on deaf ears as I began stroking my patient's cock again.
Of course, the more times I got him to the boiling point and stopped, the hornier the patient became - and the more he kept hoping the next time I started up would be the last, and he could finally shoot for me. (The guy in reality loved being edged, so this was the fun part...lol). Each time I took careful notice of the sounds and facial expressions my patient made as he approached climax; each time I stopped I noted his building desperation. Each time he would ask how much longer, and I would just remind him how important this was to him making the team.
When it got to the point that even slightly touching his boner caused pre-orgasmic reaction, I knew our exam was coming to an end. I explained it was time to collect the sample, and I held a small cup near the head of his cock. I told him only to concentrate on shooting as much as he was able.
Then, as he made some damn hot orgasmic shouts, I milked a very impressive load into the cup. From the size of the load and the power behind the squirts, there was no question he would pass the physical...:-)
(And yes, I really did this - I know it sounds like porn, lol. I'd love to do it again, too...)