Unfortunately if you can't guide kitten's thoughts another direction, he might get totally into the habit of doing this for comfort, and it might not change for a long time. Years ago, we had a kitten who would suck a rather gentle male cat we owned, who allowed it. (No matter how many sniggering jokes my dad made, we knew from observation that the kitten was not sucking the adult lower down.)
Anyway, I guess you could assume that the male likes the kitten and likes being close to him, and for some reason doesn't find the sucking annoying. I'd also assume the kitten was weaned a bit too early, thus is seeking tummy contact. Since you find it unnerving, move them apart and give the kitten some KMR whenever you see him start the suction machine. (You can get it at a pet store; the powdered kind is the most economical.) If little guy figures out that the kitten milk replacer tastes better than whatever comfort he gets from faux nursing, he *might* break the habit.
Do take seriously the need to nip this in the bud by giving the little guy a replacement that is socially acceptable. A kitty who gets used to nursing for comfort can do it for a surprisingly long time. They will even suck on their owner, like on the earlobe or the eyebrow. I used to have a female kitty who was taken from her mother at 6 weeks, and she would suck on my finger through her adulthood. Drove my husband crazy, she made loud smacking noises. (He'd say, "Is that the CAT? Eeeew!")
Consider getting the male neutered, too. Not only does their pee smell horrible when they are entire (and who needs their cat to go out "catting around" all the time, fighting and making unwanted kittens?) but maybe it will reduce his enjoyment of physical contact enough to make him less tolerant of the sucking (just guessing).
Don't get all worried this is actually quite common, the older male is just finding this soothing....no biggie