I am relieved that you don't think he has eaten anything toxic or things that could lodge or obstruct within the gastrointestinal tract since those would be emergency situations for Barclay. That said, if he cannot keep water down and dehydration is starting to arise, then I would strongly advise that he is going to need to see a vet so that he can at the very least receive anti-vomiting medication via injection.
There are oral options you can try at home but as I am sure you can appreciate vomiting of this severity will make keeping any oral medications that you might give near impossible. And when we have dogs who cannot even keep water down, we do tend to find that we need to intervene to halt the vomiting (preferably before dehydration can get any worse, as to avoid having to hospitalize him for IV fluid therapy).
Now as I am sure you can appreciate, vomiting can suddenly arise for a wide range of reasons. The most common reasons for a dog his age to show these signs are dietary indiscretion (eating something he shouldn’t have), ingestion of a foreign body (ie toys, bones, trash, etc.), toxins, viral infections (ie parvo, distemper, etc), pancreatitis. intestinal parasitism, and a bacterial gastroenteritis. If he isn't a mischievous wee soul or you are sure he couldn't have ingested anything of worry, then hopefully we can put worries like toxins and foreign bodies (which we' d want to address as soon as possible) lower on our list of concerns but it does leave a range to still consider here.
For the moment, I would advise resting his stomach from everything for the next few hours. Ideally, over that time frame, you'd want to potentially see his vet, get him started on treatment with a view to settle the vomiting and then reintroduce small volumes of water to see if he can keep that down.
In any case, if you haven’t seen further vomiting by that point, then I would advise giving him a small volume (1tbsp) of a light/easily digestible diet. Consider trying boiled chicken with rice, boiled white fish and pasta scrambled eggs (made with water and not milk), or cottage cheese with rice. There are also veterinary prescription diets that can be used in cases of gastroenteritis, (ie Hill’s I/D or Royal Canin’s sensitivity). He can have this, wait 30 minutes and if he can keep it down this can be repeated.
Once he has had an injectable anti-vomiting medication, the vet will likely give you an oral preparation to continue to control his nausea at home. But otherwise, just to note, there are a number of antacids that are available over the counter and pet friendly. The two I tend to recommend are Pepcid (LINK) or Zantac (LINK). This medication of course shouldn’t be given without consulting your vet if he does have any pre-existing conditions or is on any other medications. Ideally, it should be given about 30 minutes before food to ease his upset gut signs. Do note that if he has profuse vomiting at this stage, it will be a struggle to get any oral medication to stay down (hence why vets often start anti-vomiting medications by giving them via injection).
Overall, to hear he has such profuse vomiting, cannot keep water down, and is already showing signs of dehydration in this short time frame does ring alarm bells. Therefore, we do really want to get some anti-vomiting medication into him and have his vet have a feel of his belly to just make sure he hasn't ingested something you might not have realized. Once the vomiting can be stopped, the cause addressed and appropriate treatment initiated; you will be in a positive to address the dehydration and get him back to normal.
If your vet is not open just now, then to find an ER vet local to you, you can check HERE and @ http://www.vetlocator.com/.