Please help...

[OSusie]

I would appreciate any feedback for this situation. We have a pit bull he is 6yrs. old. About a year ago he started losing weight quickly. He also started vomiting and would have the runs every now and then. My husband took him to the Veterinarian and had a series of blood tests done and everything came out normal. We have had x-rays done also and they came out normal. We were thinking maybe there was something stuck in his digestive track. To this day we cannot figure out what is wrong with him. We have him on special dog food that his Veterinarian perscribed.It is for digestive problems. He throws up almost every night after dinner. He cannot seem to hold anything down. he does not get the runs but he does have trouble going to the rest room he strains himself, My husband said he noticed that he also strains when he urinates.He seems alert and still wants to go walking. He still barks at the men when they mow the lawn next door, but he is getting skinnier everyday. We are at wits end because we can't seem to figure out what is wrong with him. If anyone can please help us with any suggestions we will try anything to help him. He is such a loving dog it hurts me to see him this way. Thank you....