Lily - what medication is he on? Have you told his doctor about his change in behavior?
While this may be just another textbook midlife crisis - or it's possible that now that he feels better, he has the energy to change his life - but it is also possible that he's having an adverse effect from his treatment. For instance, some drugs used for Parkinson's can cause people to suddenly develop gambling problems.
Now, you probably shouldn't suggest this to your husband - he'll likely just think you are avoiding the divorce issues. But I would definitely mention it to his doctor in private. Especially if these changes are really out of character for him (did he always want to get a tattoo? Or is this a guy who hated tattoos and now suddenly he has a full sleeve?)
Also - if he's on swingers websites, please please please protect yourself from STDs, okay? He's not the man you knew right now, who knows what he's doing.
