How can you be sure if changed? Not a player? Why, you just have to wait and see. There's not much else you can do. Grab a bag of potato chips, it could take a while. I suggest Dorito's Last Call Jalapeno Poppers. Mmmm.
Open your heart, chill, and hang. What's important is how he treats you in the moment. What are his actions towards you today, not yesterday or tomorrow.
His past is his past. You had nothing to do with it. Don't judge a person by how they acted before they met you. That's as ridiculous as judging a person for how they might act in the future. You didn't know him in his past: maybe he would have shaped up sooner if he had. Or he went through some stuff back in the day he had to go through in order to grow up and fulfill his destiny. Either way, you weren't there, so you can't really size up the kind of man he was, or is. Unless you have a flux capacitor. But this isn't love advice for time travelers (check back tomorrow.)
Likewise, who he will be, and what he might do, are hypothetical situations. You know what else is a hypothetical situation? What if your boyfriend discovered a pirate's chest of gold doubloons in the backyard? OMG WHAT IF? The past is for history books, ghosts, and gossips, the future is for hopes, dreams, and robots with laser guns. The present is where all love blooms. Awwww. Puke. But true!
If your man says he's not a player anymore, if he says it's different this time, you have no choice but to trust his word. And who knows -- that trust might never, ever be betrayed. Or it might. You will just have to wait and see. This is what a relationship is, anyway. Courageously giving your trust to someone, because you have no choice. Love demands it. You could get hurt, but that's a small price to pay.
You give each other the secret answer to your heart's security question, and hope for the best.
Now... these "friends" who tell you your boyfriend is a player. What the hell is their problem? Why they gotta sabotage? Are they bitter? Angry? In love with your boyfriend?
Unless they have ironclad proof that his wang is on another lady, they should shut their back stabby mouths.