This behaviour not unusual. One of my stud cats (when I was into breeding) used to let his weaned kittens nurse on him. He was very gentle with his kittens. He didn't mind and the kittens loved him. They eventually went off to new homes. If your old boy isn't bothered by it, why should you be? If he's getting sore or bothered he'll let kitty know and that will be the end of it. Some kittens just enjoy nursing longer than others; it doesn't always mean they were "weaned too early" or he "had a bad mommy".
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