I just have a few questions first.
What is his normal diet?
Is he having any diarrhea?
Does he like to chew up things he shouldn't?
Can you look at his gums and see if they are pink and moist and press on them above the teeth and see how quickly they go from a whitish color back to the color they are. Normal is less than 2 seconds?
Can you take his temperature?
He normally eats about 5 cups of dried dog food a day, and drinks plenty of water.
He hasn't been having any diarrhea.
No, he's not a chewer. He spends his days in our dog yard which is totally fenced, and shares the yard with our 2yo black lab.
His gums seem pink and moist and the color changes quickly back to normal.
I don't know how to take his temp, or with what kind of thermometer.
You can take the temp with a digital or regular rectal thermometer. But you want to make sure it becomes the dog thermometer after that. Normal dog temperature is 100.5 to 102.5.
It is good that his gums are pink and moist and the color comes back. He should be hydrated for now. It is also good he isn't having any diarrhea and stays in a fenced in yard. I would worry with the weight loss, especially in an older dog. I would say the best thing would be for him to be seen as soon as possible. There are many things that can be going on and some of them you want to treat as soon as possible. Things like diabetes, kidney disease, or pancreatitis would benefit from beginning treatment quickly. Considering he has already not been feeling the best for a few days, I tend to lean toward having him examined soon. Also if the vomiting continues, he will become dehydrated. If that happens he will have to stay and get IV fluids because he is too large for any other way to work well.
Good luck with him. I would probably take him in tonight if possible or first thing in the morning for sure if he were mine.