Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear about Miggi's frequent vomiting up a large amount of mucous.
Is this active vomiting (retching with abdominal contractions) or does it seem to passively flow back out with not much effort (which is regurgitation)?
Does this seem to happen more after exercise or can it be any time?
Are his food and water bowls elevated or flat on the ground?
You might try elevating his food and water bowls and making him stay in an upright position (front paws up on a chair back or table) after eating and drinking to allow gravity to help pull the food and liquids down into his stomach.
Some Bulldogs have redundant folds in their esophagus as well as skin folds. This allows mucous to get trapped and can cause it to be vomited or regurgitated later.
Bulldogs can also have a herniation of the stomach up into the esophagus which blocks food from getting through properly. This is called a hiatal hernia. They can be born with varying degrees of being affected and may show no signs of it until later in life as the stomach herniates more frequently or to a greater degree. Sometimes they spit up food, other times it is just water an mucous.
If he seems to be regurgitating (passive outflow) rather than vomiting (active retching with abdominal contractions) then this can be an early sign of a condition called megaesophagus. Megaesophagus is a condition where the esophagus is dilated and has weak muscles. Food may sit in it and be passively regurgitated back up, sometimes several hours after eating.
The mucous that you are seeing may be esophageal mucous or stomach mucous and is normal, but the spitting up is not. Megaesophagus can be secondary to nerve damage from pulling on the collar/leash too much as well as nerve damage from a foreign body in the esophagus, it can also be due to a low functioning adrenal gland (addison's disease), hypothyroidism or a condition called Dysautonomia. Sometimes it can be caused by a mass in the chest affecting the nerves that control esophageal function.
Your veterinarian can do a test called a barium swallow to see how his esophagus works. He'll be given barium orally and then a series of radiographs can be taken to see how it passes through his esophagus and stomach.
And they will likely want to check some blood tests as well. I would want to check a biochemistry profile and thyroid profile, looking for things we can treat. Here is a link to a very good article about megaesophagus: http://www.marvistavet.com/html/body_megaesophagus.htmlHere is a link to read about hiatal hernias if you would like: https://www.vetinfo.com/hiatal-hernia-in-dogs.html
For now you can try using acid reducing medications to see if they help by decreasing esophageal burn and reflux, which will decrease stomach and esophageal mucous production.
You can give either:
1) Pepcid-ac (famotidine) at a dose of one 10mg tab per 20 to 40 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one 20mg tab per 40 to 80 pounds of body weight every 24 hours.
These are acid reducers and should help settle his stomach and decrease the reflux if this is related to gastroesophageal reflux. They are very safe and can be used as needed with him.
I would also recommend feeding him in an upright position and having him stay in an upright position after eating or drinking for at least 15 minutes or so after eating and drinking.
Finally if he is a fellow that tends to drink a lot quickly I would purchase a bowl to help slow down his drinking. See this link for examples: https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=bowl+to+slow+down+dog+drinking&tbm=shop
Best of luck with your Miggy, please let me know if you have any further questions.