He should stop taking the kittens milk, but sounds like he’s being a good older brother, I’ve had several cats that weirdly do this at times. You have to make sure he doesn’t hog the milk, he may just be very insecure, that’s why he’s still trying to nurse. He also must b fry young and had a mothering instinct, the laying next to the kittens keeps them warm and secure while moms away. He he starts trying to chase her off the kittens or you notice the kittens are getting thin, he will have to be taken away from mom and the brood so she can raise them, otherwise they should be fine like this. Many males that are neutered at a very young age also instinctively will nurture kittens, I’m not sure if it’s because they didn’t have a full amount of testosterone running through them or it’s a nature nurture thing.
Occasionally the males do this, more often than not the young females do this, especially if momma cat had litters back to back, first litter barely weaned when next litter drops, I’ve seen momma nurse the older kittens with the newborns at the same time. Again wired but not unheard of.