There's a dark view of the term "let's just be friends.
"It's too associated with lies and insincerity, and while it's true that a lot of people use it as a way to get out of a relationship and never call you again, there IS another situation and it's more common than you think.
Yes, it's true...
sometimes when a person says "let's just be friends," they mean it and really still want you in their lives.
Actually, this doesn't always mean he wants you in his life as a friend...
sometimes it's actually a way to keep you in his life at all while he figures out how he really feels about you.
Relationships have a lot of ups and downs, and sometimes people question how they really feel about their partners and how committed they're really willing to be to that relationship.
This is natural, something humans do...
but it can be very confusing and leads to a lot of breakups - breakups that neither person really wants to happen.
Your ex may be struggling with just this kind of situation, and if he is then you have a pretty easy job of getting him back.
You just have to play your cards right, and while that may not always be crazy simple you DO have the odds in your favor.
If he really still cares about you (and there are ways to tell), the "let's just be friends" zone is just a springboard to bringing things back together.