Hi there, sorry to hear that Banjo passed away.
There are many factors to consider when a Horned lizard
"poops out".
Obviously, to keep any HL alive for any period of time, he is going to need a good heat source. They need it very hot in their basking spot (around 105 F), and room in their enclosure to escape the heat as necessary(around 85F).
another thing: is he eating enough? Sometimes a lizard will flat out refuse to eat, appear healthy, and then just deteriorate until they die. Put a few (2-5) ants in at a time. Gauge his response to offered food.
Also, think about this time of year. If Maximus (cool name BTW) is a WC, he'd be getting ready to hibernate right now. It is my opinon that even when kept at normal temperatures, a lizard will still become lethargic when its biological clock says "hibernate". Check some links to cable's website to find out how to accommodate him if he is entering a hibernative state.
Make sure he has enough water as well. Too many people overlook this neccessity when considering desert lizards.
If all else fails, take him to a vet.
I hope he perks up. Good luck with your HLs, and let us know how things turn out.
-Phil