Once again, he misunderstands how disability law works. NBC can't STOP him from auditioning, but unless they say "no you can't be on the show because you're disabled" he really doesn't have a case. Russ thinks his disability means people have to use lower standards in evaluating him/his work but that's not how it works.
EDIT: Russ's message to Sam(f) indicated he was genuinely shocked that Null got a lawyer. I think he was expecting Null to settle in order to make this go away. Then Null goes off-script by not only hiring a lawyer, but hiring the very man that's beaten him twice before. Russ hates it when people stand up to him. He was seething over Erika using the law against him, and now he's pissed again that someone isn't bowing to his whims. And oh yeah, he thinks he should have qualified, therefore it's discrimination, when in reality, anyone with any musical talent would say he's awful without factoring in his disability.