I think a rebound relationship is a new relationship immediately after (or even during the ending of a previous one) a significant partnership.
People don't always know that is what they are involved in until some time has passed and some healing and clarity is achieved after the ending of the significant partnership.
Myself, I have involved a man in a relationship before I was honestly ready to be in it. What I mean is that he was the rebound relationship for me. There were times he would say to me that I wasn't ready and I would swear up and down that I loved him and knew how I felt etc. It was true at the time, I'm not the sort of person to emotionally trick or deceive somebody.
One day, and it was about 6 months into the relationship, I finally stopped running (from myself as it were)...and had to admit that I was a mess and needed to be by myself and not drag anybody along for the ride. I broke it off. As I said, I'm not one to deceive anyone. I've ended many relationships in a bid to be emotionally honest to myself and others. I do not cling to relationships rather than be alone.
I feel awful about it. I didn't do it intentionally. What I have learned is that you need AT LEAST a year, probably two, learning who you are again as a single person, before getting emotionally involved with another person. It may even take longer for the person who was the partner who was left in the significant long-term relationship as they have to deal with abandonment and loss - if you are person ending the relationship you have often processed a lot of the feelings and are somewhat in control of what you do. Throw into the mix property settlements and maybe children and there's a big mess to untangle even if it's something that is ultimately good for you. The ties that bind are difficult to sever. Habits and familiarity are very seductive.
If I were you, I would take him at his word. Say thank you, and tell him to call you in 3 months to see how things are travelling. If you are meant to be together, taking a hiatus for 12 weeks won't hurt. If neither of you are sure at that point, take another 12 weeks.
Good luck to you.
edited to add - I have often heard people talking about a book called Surviving your Boyfriend's Divorce (or something like that) and they have found it very helpful. Maybe it could give you some insight into what he is going through.