There's no bar on YOUR end here. You can do whatever you want.
On his end, his actions may be explicitly constrained by a written code of ethics in the profession. Even if not, his actions will most definitely be constrained by an UNwritten code of ethics. There's a lot of tacit rules, in that there ivory tower.
Honestly, I don't think he would accept yr overtures -- even if he wanted to, really really bad -- unless he's considering a wholesale career change, out of academia altogether.
I think it looks bad on his end so don't be surprised if he rejects you.
They are held at a higher standard and although he won't get fired, if word spreads he's willing to date students, even former students, schools and universities typically frown on that if he's ever looking for a new job.