I have a male cat that loves kittens. He would bring them home from the neighbor's litter, then take them back later. Momcat wasn't concerned, I guess she knew he wouldn't hurt them. He would lay in the box and watch over them while mom had to go eat or potty. When we had a cat that was in labor he lay in the box with her and licked her face while she was giving birth. He helped clean them and grieved right along with her when none of them survived. He has raised every kitten I have ever brought home. He would have mad a great mommy. Maybe NaNa just likes kittens. Does the mom have a problem with him doing this?