Has he been eating ok?
Whatever -eating or not, to lose weight there has to be a reason, and if he hasn't been eating then there's a reason for that too. Pain can put a dog off his food, obviously nausea can. Also I have known dogs who disliked dry food every day. Unfortunately, they can't tell us what's the matter.
My suggestion is take him to the vet, and get his bloodwork done, for starters. Bloodwork can tell a lot, whether there is liver or kidney imbalance, whether there is diabetes, infection, etc.
This dog needs to be seen by a vet to determine the cause of his weight loss. For the muscles in the head to be affected he has to be extremely emaciated, which points to several possible (very) serious illnesses or him being grossly underfed. Please don't wait, take him to the vet ASAP. If you wait too long and, for example, it turns out he is diabetic, he could be in diabetic ketoacidosis. When this happens, the body cannot metabolize sugar as a food source so the body uses its own fat as fuel instead. Please let us know what the vet says.
He is eating good, pooping regularly. Barking, running etc. I think his hips are bothering him. She recently got 2 kittens. He is extremly jealous. So i wonder if its stress. As well as a little arthritis.
Well, as he is eating properly, then the weight loss remains unexplained. It is definitely time to get him checked, and the cause of his weight loss found. Like Ghilly says, if it IS something like diabetes, the quicker he gets treatment, the better. I wouldn't blame stress because of the kittens. That shouldn't cause DRAMATIC weight loss in a dog who is eating well. There has to be a physical cause.