If past concussions are affecting his health and ability to live a normal and happy life, he should retire before he could make it worse by playing more hockey in the NHL. Not to say you can't get concussions by accident during day to day life, but playing hockey I think is safe to assume is riskier in that regard.
He should have enough money. His health and family should now come first if concussions are starting to affect him. But obviously it's his life and he can do what he want. We'll see, but I have a feeling he retires.
If he does sign with someone, it'll probably be in the 5-6m dollar range, IMO. Maybe less if he wants to play for a contender with limited cap space, so could be in the 4m range if that's the case. I know E. Kane got more, but teams will be wary of his health issues and recent production, plus I can't see him signing with a team that's not considered in the running for the Cup, and most, if not all, of those teams have pretty limited cap space. Not to mention he'd probably have a limited amount of cities he'd consider playing in, and he'd want to make sure he fits in well with the team and style of play, so that limits the options further again.