Getting his account back? No. Never.
As far as letting him stream LoL? Yes. Maybe.
But it would exceptionally difficult.
He can't stream or Riot will find out and ban him. So he has to stay low, which will make it difficult to prove his reform. He would have to wait till he has a huge history of clean play on an account, and if Riot finds even a hint that he has been toxic on that or other accounts it would undermine the attempt.
He would have to play cleanly, at a minimum, for the same amount of time he spent being toxic. But likely for much longer than that, to genuinely prove he has reformed. Considering that, from what I understand, he has been defiantly toxic for 5 years...
It would be a monumental effort on his part, and would likely not see the results for years to come.