Some neutered male cats do like to 'adopt' kittens and play with them or look after them. It is instinct for cats to pick up kittens by the scruff of the neck - it actually calms the kittens so they don't struggle. As long as he is just moving them (especially away from trouble, like a fight, I don't think you need to worry. My Wellington disciplines the other cats sometimes by putting a paw on them if they are fighting - it is all part of teaching manners.