With all due respect to everyone posting above about the door frame pull up bars, those things are junk and
the inside door frame of a house is too short.
If you are following Jocko Willinks advice, and I agree with it, build him one in the garage or back yard with materials from home depot. (Jocko even says not to buy the door frame kind)
Build it tall enough so that, as he grows it won't be too short for him. (ones inside the house in a door frame tend to be too short for realistic pullups)
You don't have to get too crazy with it,
I use one at the gym or at a local park, but if I were going build one for hard use, I'd put it in the back with
4x4s and a 2inch pipe as a bar. Do not let him put tape on it, even in the summer, calluses are good.
You can use a milk crate to help him reach the bar if he needs it. Height of the bar needs to be high enough so that he can not touch the ground and he won't have to bend his legs at the knees to keep himself from touching like you have do to with a door frame version.
Have him do a couple every day or every other day and he will build up to it. I do thiem in sets of 5, typically, with about one minute rest between and keep going until failure. Doing negatives on the "down" is a great way to build as well. Don't let him just flop back down. It needs to be controlled.
Pull ups were very hard to tackle for me, so I made it a personal challenge and something I could build up to.
Now I love doing them.
Also, and I can not over emphasize this enough, the fish-wiggle/kipping/joke of a so called pull up that you see
cross fitters doing....is NOT a pull up. A pull up is, pulling yourself up until your chin clears the bar, down to a dead hang and back up.
