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!