Well the way I go about dealing with this kind of thing.. and.. this may seem slutty, but I just make myself available. You don't have to try to be with the person officially, but just by being in his presence more could get some sort of reaction from him in the right direction. Smothering is different though, when I find someone I like, as I never make the first move, I just become part of his environment more.
A person can tell when they're destined to be a rebound, I learned that the hard way with my latest train wreck relationship. When they fall hard and fast right after a break up, then they're an obvious rebound, it does feel good at the time as you're actually seeing them at their best as they're trying to repress their worst to try and recover from the loss of a relationship. Then as time goes by you see their worst because there isn't anything maintaining the relationship.
Now you said they were together for three years? That's an extended period of time so I suggest that you move slowly, I say he's not looking for a love interest, rebounds usually happen spontaneously.
So to sum it all up and to finally make sense, just be around, but not with benefits attached. He wants a friend, in the long run not a rebound and if you become his rebound you probably won't last long as his last relationship lasted a long time, which means he's looking for a relationship with substance or he'd not be in such a long relationship regardless of who dumped who. No one can really put it into days, weeks, months, years of waiting for him. You kind of have to know the person, if their body language, attire, demeanor, personality, etc. has changed since the break up, then it's probably safe to say he's not ready for another long standing relationship. If he seems similar to what he was during the relationship with his ex, then it may be the appropriate time to make some sort of gesture as he's more himself.
Krista