Mixed signals from older dog to 7 month old puppy?

Hi!

I have a 7 year old corgi mix and an almost 7 month old corgi. The puppy is very energetic and loves other dogs. My older dog typically likes other dogs. He has been very on and off with the puppy from the start. He would “protect” the puppy from our oldest dog (who has since passed away as of last week). But then turns around and gets mad at the puppy a little later over seemingly nothing.

We just moved two weeks ago and lost our oldest dog. Our older dog is now getting aggressive towards the puppy. What are ways I can mitigate this? We remove food and treats so there is nothing to fight over, but the older dog still is (sometimes) very angry over the dog. I have been very loving on our older dog to help him deal with all the changes. Puppy also gets multiple naps a day so the other has breaks from him. He is not neutered yet.

Thank you for any advice!