First, a constipated pet will generally not visually appear bloated. For the belly to actually be distended and/or firm is very concerning.
The appearance of a bloated belly could be caused by:
1. Stomach Bloat
2. Stomach Torsion (a emergency) when the stomach twists on itself.
4. Worms (not really that common in older pets)
5. Organomegaly (Enlargement of the Spleen, Liver). VERY common
6. Tumor in the abdomen
Unfortunately, the loss of appetite has too many causes to list. Anything that could cause nausea or pain would be a consideration.
The vomiting elevates our level of concern. I do truly feel a vet visit is a must. No matter what, this will not resolve with a wait and see approach. I will mention some antacid therapies to help any nausea, but it is critical to get him evaluated. If the vet agrees about the belly, I would strongly recommend X-rays ASAP.
To help settle the stomach you can use of the following, but not as a replacement for veterinary examination include
1.Pepcid A.C. (famotidine) comes in 10mg, 20mg, or 40mg tablets.
You can give it every 12 hours. You can give 0.5mg per pound of body weight. So, a 20 pound dog would get 10mg.
2.Prilosec (omeprazole). It comes in 10mg or 20mg tablets.
You can give in every 24 hours. You give 0.5mg per pound of body weight. So, a 20 pound dog would get 10mg
3.Zantac (Ranitidne). It comes in 75mg, 150mg, or 300mg sizes.
You can give it every 8 to 12 hours. You give 0.25 to 1mg per pound of body weight. So, a 20 pound dog would get roughly 1/3 tablet of the 75mg. Even with bigger pets, it is easiest to get the smallest size tablet. Even a 75 pound dog would only need one 75mg tablet.
Very sorry about how things are going. Hope that information helps.