Well, he's told you and he's added you to FB. Both good. See how he posts to/about you.
It's, unfortunately, not uncommon for siblings/parents to 'stay out of it' if someone is having an affair whilst working/living apart for a time, so it's not a guarantee that's he's above board.
Can you see HER FB page? Does it indicate that SHE thinks she's single?
However, even if you ascertain that he's now genuinely single, it doesn't alter the fact that you 'don't want to be a rebound'. There are no guarantees in life! You take your chances whether they've been single 5 mins or 5 years. If you aren't ready for that risk, then you shouldn't be 'dating' yet, give it more time.
'Dating'. Feels like such an old fashioned word 😁
Does he still love her? Probably. Will he always love her? Probably. But it doesn't matter, it's separate from what he can feel about someone else, in the very same way you can love your Mum & your Dad, loving one doesn't take away from the other.
It's only an issue if he's still 'in love' with her, but only time will tell.
If you are still 'not over' your ex, then are you being fair on him? If you think it's ok to 'date' him to get over your ex, why isn't it ok for him to 'date' you to get over his ex?