What exactly looks suspicious (beside him waking up after few years break) as a change of email and password is not much of evidence. Has he changed the boards where he is usually writing, or maybe his post quality suddenly changed? You should put a little bit more effort and analyze his activity before asking others to check it out.
I got a message few days ago when signing up in a website("A Data Breach on a Site or App Exposed Your Password"). So i have decided to changes my primary email address and password to every site i registered for. I left BTT a long time ago so after changing password i thought why not make some post again. I know a lot better than 2018 so may be i can make something for myself and others. It's been a long time so it's not unusual that i can write in more matured way than before as my knowledge expands a lot. Haven't thought that this will caught some peoples attention.
I can send a message from my old registered email address("I do not want to share it publicly') to any moderator if i really needs to prove my account ownership. I do not know what more to say and i think it is not any violation of rules to be active after a long time.