Never, and I mean NEVER, leave them alone together. As much as you may trust your new dog, and as calm as he may be around your step-son, you never know what may happen in a room where they are alone together. I say this from personal experience, and believe me, no one wants to deal with what happened to me.
Long story short, I had a well adjusted CLM who never showed any aggression. I hunted with him one morning and came home, dropped him off, and went in to my office. He apparently was asleep in my closet, where he normally slept, when something happened with the 6 year old friend of my son's when they were playing in my bedroom. Don't know to this day the specifics, but the result was several gashes in the boys face. I suspect that my dog was sound asleep and the boy jumped on him or otherwise surprised him. Thankfully, the gashes did not affect the boy's eyes and all wounds healed with no scarring at all. My wife ended up hating that dog, even tho he never showed any aggressive tendencies in his life again. Believe me, it was not easy having him around with my wife hating him.
I know you were looking for advice on enjoying your dog with your stepson, and that is totally possible with advice others have given you. Best of luck... but always keep this advice in mind. Young kids and dogs left alone do not mix.