First things first prepare a work space, get some q-tips, some water or coconut oil. Put the coconut oil/water in a container and put the ends of a couple q-tips in it, have some paper towel to put the dirty q-tips onto when you are done with them, then get a chair and sit down with him. Put his back against your stomach and move his bottom forward a little bit between your legs.
Next you need to expose his penis. To do this feel for the firm line under the skin that is his penis, push your finger down where this line ends (furthest away from the hole), then roll your finger down, I personally find it easiest to do this with my thumb. Then use the q-tip to wipe away the gunk with your other hand. If it is wrapped around the penis you may have to take a little bit of time or even unwrap it with your fingers (if the later turns out to be true you may also want to have on disposable gloves).