I coached several little league teams and this one came from the stands constantly. Kids would lift one elbow, then the other until they couldn't even move, much less swing. If you watch pros, you often see the elbows actually down close to the body. I give up.
For a right-handed hitter. My suggestions are reversed for a left-handed hitter.
Grab a bat, and get into your stance. Then let go of the bat with your left hand and see what happens with when you see the track your right elbow has to take to get to the ball. Major league hitters start from every position imaginable, yet, all the good ones are at the same spot when the bat is meeting the ball.
Doing that will automatically guarantee you have an A-B-C swing instead of an A-C swing. Are you gonna have a loop, or are you gonna have a chop down to the ball? A good coach will teach you a good A-C swing. A-B-C means you are going around an arc with the bat, rather than top to launch.
If you don't have a swing that begins with getting the knob of the bat to your left hip before the barrel drops, you aren't gonna hit the ball very well.
Something that is REALLY IMPORTANT. Keep their eyes LEVEL. So many kids tilt their heads and they only see the ball with their left eye and the right eye is partially unable to see the full flight of the ball because of your nose. Make sure your son is SEEING the ball with both eyes.
If you want to experiment, have your son get in his hitting stance, then have him freeze, then take your left hand and cover his left eye, and ask him to tell you how much of a baseball he could ACTUALLY see. Try it yourself.
The left-hand pulls the bat, and the right-hand guides the bat. As soon as you straighten out the left arm you have reached zero acceleration.
Any good hitting coach should start at the feet and go up. If the waist to the eyes is good, and the feet to the waist are bad, there will be a problem.
Swinging more than 12 times in succession is not a good idea. Even if you have to break up the lesson and use a cut-down-to-size broom handle, it will give your muscles a breather between the exertion of full swings.
Find a coach that will teach you a major league swing. Your son doesn't have to be a major leaguer to have a major league swing. If a coach isn't able to give your son his undivided attention, that coach is not the right one for you.