They are both male. I know sometimes he wants to initiate play, but other times I think he's just getting in his face and barking at him to assert himself. Anytime the other dog has a toy/item of interest, he takes it from him. He nips at his tail and just annoys the other dog trying to get a reaction out of him. Sometimes they do play and I can tell that it's a mutual engagement. I'm not sure if this is just a puppy thing, if it's something that will pass, or if it's something that I should address immediately. He's in puppy kindergarten and does not act dominant there, so I don't know if it's just him establishing himself as the alpha in a common space or what--any/all advice is welcome.

My Sheba did that as a puppy as well. Kennedy would let Sheba do whatever she wanted and would let her push her around. Prancer (my Standard Poodle) did not let Sheba get away with behavior like that, and I only wish he could have stayed with us. Sheba NEEDED to be taught that wasn't okay, and I didn't teach her. I left it up to Kennedy to decide when enough was enough, but Kennedy isn't the type of dog to be assertive.

In my experience, nip it in the butt. Since I didn't stop it then, Sheba thought it was acceptable. Now Sheba is 8 months old and she is constantly trying to take anything and everything away from Kennedy and she is displaying some really unfavorable behaviors because of it, and now Kennedy is to the point where she knows Sheba isn't a puppy, so now they fight. If your dog isn't going to show the pup that what he is doing is annoying and not okay, then YOU need to show him. I learned that way too late, and now we have major fall out. Don't let your puppy get away with these behavior.

I thought it was just "dogs being dogs" and I needed to "let the dogs work it out". NOPE! Put a stop to his behavior while he is young.

Thanks for the advice. So how do I teach him to play in an acceptable way? Do I redirect the behavior after saying "NO" or divert his attention to a toy?

If he's not responding to redirection or verbal reprimand, he needs to be put up. Rodeo spent lots of time in timeout as a pup because he was trying to be a jerk.

Yes that is exactly it! Make sure if you are crate training him not to use his crate as a time out. The sooner you show him that his behavior is not okay, the sooner he will stop and your house will go back to being some out in control .