I am sorry to hear about your dog.
My primary concern is the persistent lethargy and unwillingness to eat. Vomiting, unfortunately, is an indicator for just too many different problems. We just don't know if this is a dietary indiscretion (ate something you do not know about), a foreign body, other organ problem, etc.
Therefore, even if the vomiting has subsided, we know that the problem persists. We really should get some abdominal x-rays, and I would strongly recommend a basic blood test. This is a good test for an overall health assessment anyway for a 8 year old dog.
There are likely some injections that can be given to help settle that stomach. Depending on a recheck examination, it may be necessary to hospitalize him for a day or two.
To help settle the stomach you could give Pepcid (an over-the-counter antacid). You can give 10mg per 40 pounds of body weight. So, a 10 pound dog would get a ¼ tablet of 10mg. Pepcid comes in 10mg or 20mg tablets. You can give it every 12 hours.
I have included a pretty comprehensive link below for a little more information.
Please let me know if I can be of any further assistance.