Dog angry over other dog's play

[GrumpyDog]

To begin, I have 4 dogs. A 7 year old minpin/yorkie mix, a 5 year old minpin, a 3 year old doberman, and a 9 month old golden retriever/lab mix. My oldest, Roger, has always been a handful. I got him when I was in college and he was my first experience training a dog. He suffered from horrible separation anxiety, couldn't be crated because he's soil the crate, couldn't be left out of the crate because he'd destroy the house. Needless to say, it was A LOT of work. He has since become a wonderful member of the family, and is functioning quite well.

Then in March of this year a puppy showed up at our house with a broken leg. He'd been run over by a car. We took him to the vet, got him fixed up, and that's how I ended up with the golden retriever mix, Dodge. Dodge and Krueger (doberman) have become best friends. Krueger finally has someone her size she can really play and wrestle with. The problem is, this makes Roger VERY mad. When they play he just barks and screams at them nonstop. It's definitely not a play bark either. He is angry and distressed and I just don't know why it bothers him so much. He does this when any dogs are playing. He recently visited my mother and she had to pick up all the dog toys so none of the dogs could play.

I've been trying to teach him to be quiet on command, and it seems to be helping a little. Mostly because I have treats and he'll focus on them rather than the other dogs. Does anyone have any other suggestions? Why would he be acting this way?

Here's a picture where you can see the big dogs playing, and Roger trying to tell them not to.