My dog seems sick, vet said hes healthy even though skinny?

i have a 4 year old rott/pit mix, he started acting weird couple weeks ago, he was not himself and still isnt, he crashes into everything, seems lost and dizzy, can hardly eat its like he cant move his tongue, he doesnt bark or cry at all now, he just growls a little at other dogs,,he doest open his mouth wide, its either shut or opened for a quick sec to try to get food but he doesnt grab it, i gotta feed him by hand but he still has a hard time eating, i think his condition is treatable by havin surgery, neurosurgery,,,i cant afford to pay at once but i can if i could do paymens,, i dont want to put him down, has anyone had this problem with their dog or know how much it would all cost? I know its in the thousands. Thank you