Jo,
I doubt that little bit of paint is doing anything at all. If it's been sitting as long as you said, the gas will dissolve that stuff pretty quick.
Check the tank vent on the cap.
If it's clogged up, it puts the tank in a vacuum state and has the effects that you are describing.
It'll run fine under light throttle, as it can still get a bit of air in the tank, but once you open it up it can't pull the pull the fuel.
It'll show up more if his bike is a carb model vs. a FI model.
Another kinda obvious thing is to check the air filter.
This is no BS......
I was out riding my Grizzly for almost a week strait, up in Tenessee. Ran great the whole time. Came home cleaned it up, put it away.
About 2 or 3 days later, I go out to take it for a quick ride and it would run, but didn't have any throttle response or power. WTF?? I check to make sure I had gas, the fuel shutoff was on, choke wasn't on, etc.
I shut it off, pulled the air box and, HOLY CRAP, a friggin mouse got inside the airbox, built a nest and damn near ate through my airfilter in the process. It was using my airfilter as it's nest.
I could just see myself trying to fish mouse parts outta my engine.