It all comes down to core strength and timing. If you don’t know how to do a kip, you can’t teach the technique. You can get him a kip bar or a pull up bar for him to use to work on leg lifts and l holds. Or you can do core-strengthening workout videos together. It can also be helpful to watch videos teaching the skill on youtube. Watching it done correctly in slow motion helped my son quite a bit. The other big thing is to reassure him that it will come. I saw an interview with a bunch of Olympic gymnasts asking them what skill was the hardest for them to learn in their entire gymnastics careers and the most common answer was the kip! It’s hard!
I have mild autism which makes me like a big kid at heart but I LOVE hanging upside down and gymnastics sooooooo much and these are my YouTube videos which are cartoon gymnastics comics made by me so require reading! I'll link my two playlists that will show all my videos I made in that playlist and my channel itself! My one playlist gas 21 shorts and the other 15 shorts all under a minute long except one video that's under two minutes!