Betta Acts Sick But Can't Figure Out What

[Countrylovin4evr]

My mom has a betta in a 5 gal tank. Water permeters are great and usually I'm good at figuring out whats wrong. I also know most bettas when dying tip on nose.
This one is acting funny. For months now she says he has just laid on bottom of tank on his side. He rarely moves, never coming up for food. She has thought him dead many many times and will reach in to check and find he'll swim off only to come right back and lie on floor again. He's a brilliant red but is starting to fade in color. She also has live plants and floating plants for him.
 I suggested seeing if he was bored and for a temp testing had her move 2 guppies into tank just to see if this once fiesty betta would perk up and even try to attack the guppies. Nope. Nothing. Not even a blink from the betta. So of course guppies are back to their own tank but I thought it worth a try to spring him to action. I have suggested euthenizing if he is as week as he appears. But he seems to be fighting and so we don't want to kill a fish that just prefers to sit on the bottom. 
Any suggestion of what is up and what to do?

 

[ReiRei]

Same problem here. Does it have LOTS of plants? I just put 9 plants in with mine and suddenly he was alive again.

 

[Wildbetta]

Does he tank have a heater?  If so, what is the temperature?  When you say all the water parameters are good; what are they being tested with (strips or liquid) and what are the readings in numerical form?  In the meantime, fashioning something for him to lay in closer to the surface of the water would help him out a whole lot so that he doesn't have to struggle to the surface to breathe.  Also lowering the water level is another way to achieve the same thing.