Poor little guy

. What about any other symptoms, even random ones? I seriously doubt it's this, but I'm linking you to info about Cushings...the only reason I mention it is bc weight gain (pot belly), and lethargy can be symptoms...and Cushings often goes undiagnosed. There are a whole bunch of other symptoms too, so maybe just give it a look (symptom list is about 1/3rd down the page):
Cushings
The irritability he has makes me wonder about pain somewhere too. Is he still playing with his toys?