Seems like his behavior is working... He keeps his legs straight, he gets to play video games all day.
To be fair, this is pretty extreme behavior just to get you to back off, so I would certainly consider having him evaluated, but I would be careful about "letting him do what he wants" because of not bending his legs. You may get some backlash, but (depending on where you live) there should be supports (e.g. a psych crisis line) if his behaviors are extreme enough to be a safety risk.
At face value, it doesn't sound like an Autism Spectrum Disorder, but I couldn't rule it out based on the limited info you've provided.
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