I got my female staffordshire bull terrier when she was 8 months old and had the same, and other issues with her. These breeds of dogs are very affectionate, loving, caring dogs bred for big game hunting and unfortunately, yes, fighting. It is their natural instinct to love, play, and wrestle/fight as well as defend who they love who loves them.
How I trained my furbaby to defend my home was pretty easy, she caught on quick but it took some time as well as me allowing her to mature and know and understand what her territory and home was. How I did it was repeatedly show her by making her sit at the edges of the property, pointing to a line or drawing one in the air and saying "gaurd", "home". Not just once in a while - every single time until she started getting it.
Charlie will naturally and instinctively get this too as he matures. It will help to wrestle with him and play ball with him, don't always throw the ball kick it too so he learns that us humans can also use our feet and he'll learn how to "milk shake" (defend and respond offensively when he needs to) to someone kicking at him. Right now he is still a puppy and will be until he's full grown when he's around 2 years old. Now is the best time for training. My girl is now going to be 6 but she's extremely intelligent, very affectionate, and because of how I repeatedly-every-single-time trained her she is VERY responsive to commands. If someone says the trigger words "Gaurd" or "Home" (I've also trained her with those words in other languages) IT. IS. --->ON<--- when she hears them. She will get up like a guard dog at the gates of hell and all-but take the door down when someone she doesn't know knocks, when someone new or a stranger comes within the boundries we set wheather she's inside or out it's the same thing - she lets them know she's there, lets me know they are there, and scares the soil out of them if she needs to but she also immediately quiets and sits on command.
Play with him. Playing is a form of training that teaches an animal how to fight, defend themselves, hunt, guard, and react to situations they come upon. Ever see Lions "playing"? Same thing. Use Pavlov's theory of repetitive response.
I hope this helps, and helps you all including Charlie become the family and dog you wish for. Pit bulls have a bad name because some people are just bad owners. Good or bad an animals behavior is not the animals fault, it is the owners.
There is also always the option of getting him professionally trained. Which really isn't all that expensive. Just, please, if you do entertain the suggestion of having him professionally trained, do your homework on the trainers your prospecting and do your research on that person thoroughly with #1 being Charlie's and #2 being you and your family's best interest.
I could go more in to detail and give you more detailed info of how I got my baby girl to be the way she is, she is absolutely amazing! I've actually started training her to sit and growl a few feet from the door now because the way I've trained her previously I realize now isn't the way I want her to act when someone knocks. I don't want her taking the door down because a package being delivered needs a signature. So I'm now making her sit (I started doing it by holding her there with one hand on her chest "stay" - now she's at the point after just a few times of doing it that she sits where I want her to with "Stay!") But I think you get it. Be Charlie's Alpha. And mold him into the dog you want him to and know he can be. Bull breeds are the best dogs in the world in my opinion, through experience. They are very versatile and adaptive.
https://en.wikipedia.org/wiki/Pit_bull