BC puppy herding other dog?

[SumitK]

So I recently got a 10 week old border collie puppy named Kylo who is a pretty quick learner. He is a bit nippy like all puppies, even more so than my now one year old pomsky (Eevee) was when I got her. He is learning not to bite people, as I've been redirecting with toys and yelping in pain when he does nip - both of which worked with Eevee and seem to be working with him.

The issue I'm facing is that when Kylo is outside of his pen for playtime with Eevee, or when we walk outside with the two dogs together, he tends to chase after her and nip her neck. It seems like it's not a hard bite, as Eevee usually just growls at him or will tap him on the head with her paw to put him in his place.

Eevee is generally a very submissive dog, and I can tell this starts to annoy her, especially when walking outside, as he'll chase after her and nip at her neck while she's walking almost constantly. When this happens she tends to try to ignore it.

Is this him trying to herd her? I'm trying to understand if this is his herding instinct and what can be done to nip it in the bud so it does not become worse causing an issue with other dogs.

I've also noticed he's not very food oriented, but this may just be that the treats I have aren't very high reward to him. As a puppy I could lead Eevee with a treat in front of her nose, but with Kylo the second it moves away he just goes back to whatever he was doing.

I live in an apartment, and most of the time Kylo stays in an x-pen while Eevee can free roam. I only let him out to play with her for a few 20 minute sessions in a day, and he tends to gun straight for her toys (which she allows, but seems to not want the toys anymore). He drinks from her water bowl which she also does not seem to mind as long as she's not trying to herself. He's coming along well with potty training, he'll pee on a pad most of the time, with the rest being outside, and has been pooping outside every time. (I take them both out every 4-5 hours)

Any advice is appreciated!