Cats who suckle like this are not uncommon, although mostly they suck on cloth, especially wool. We had one who had a fixation on ear lobes, and my earlobes got very raw as a result! There are several theories about sucking behaviour. Cats with Oriental breeds in their ancestry are often sucklers -- so it could have a genetic component. Many cats who were weaned too early or too suddenly will continue to suckle on their human companions. It is also associated with anxiety and stress. My guess, since who can know with a stray, is that your cat was probably weaned early, possibly because the mother was malnourished or killed, and has transferred the behaviour to you, his new mother figure. If you want to discourage him, there are stages. First, when he starts, bop him on the nose, gently but firmly, and say NO. If that does not work in a couple of weeks, start putting an unpleasant tasting substance on the places he likes to suck when you are with him -- citrus oils like lemon or orange are very bad tasting to cats but smell pleasant to humans, so they are a good bet. If that does not work after a couple of weeks, then get a small water pistol and squirt him in the face when he starts up. In some cats, the compulsion is so strong that nothing short of medication works. Personally, I would rather not go that route, but your mileage may vary. In all cases, make sure you balance the discouragement with some positive behaviour -- your cat is suckling because you are his special one and he is demonstrating his love and need for you. Just discouraging him can confuse and depress him, so make sure to add exxtra playtime or strokes to make up for the activity he is losing. BTW, the lemon oil worked for us, but it took a couple of months before we could go around the house without it.