Hi there, you are through to Dr. Jake. Im a UK based vet. Im sorry to hear of your concerns with your dog Max.
Vomiting is a common symptom in dogs that we see on a regular basis. it can be caused by many underlying conditions. which i will explain below.
The most common cause of vomiting in dogs is gastritis or gastroenteritis. this causes vomiting and or diarrhoea. once of the most common reasons for this is dietary indiscretion- they have eaten something that the body doesnt agree with.
another common cause of vomiting in dogs is ingestion of an object that isnt digestable and leads to a blockage of the bowel. foreign bodies can lead to regular vomiting, not wanting to eat, lethargy and abdominal pain. this is generally seen in young dogs, puppies or dogs prone to scavenging undigestable items.
another condition called pancreatitis is common in dogs which is inflammation at the organ, the pancreas - this is involved in digestion and often gets inflamed by, again, eating food which is high in fat. this also causes vomiting +/- diarrhoea and abdominal pain which can result in lethargy, dogs can be hunched over or try and stretch out alot due to abdominal pain.
other conditions such as liver and kidney disease can cause vomiting though this doesnt cause acute vomiting as in max's case.
as he has been vomiting a lot and is lethargic i would recommend an appointment with a vet today for a full examination. they will be able to conduct a full examination, making sure there is no signs of dehydration, they will be able to feel his abdomen to make sure there is no pain here and possibly feel if a blockage is present. the vet may recommend blood tests and imaging in the form of x rays or ultrasound if they feel necessary and can provide medications to stop the vomiting.
due to him being in the older age bracket it would be a good idea for him to be seen as they can deteriorate quickly if vomiting regularly.
i hope all this information helps