I can understand your concern – it sounds like Maverick has been through so much in such a short time.
From your description, Maverick’s symptoms appear to be non-specific and general with several body systems being affected which means that the list of possible causes is quite long. His multiple veterinary visits would have included a thorough physical examination and blood test at the time of his relapses and as you mentioned nothing specific was found. This can be frustrating, especially with veterinary patients who can’t tell you how they feel but your vets plan to perform further testing with an abdominal ultrasound and endoscopy sounds like the best next step.
Often, solving a challenging case such as Maverick’s involves a process of elimination so even if a test comes back as ‘normal’ it can still provide us with a lot of information and greatly assist in arriving at a final diagnosis and treatment plan.
Unfortunately, challenging cases can be costly to investigate and this is one of the difficulties we face as veterinarians everyday. The professional advise you have received sounds excellent so I would hang in there and continue with further testing to get to the bottom of little Maverick’s problem.
Best of luck.