It is very good your going to teach him to sew. Being a designer, you do have to be able to sew, as well as design the costume/outfit. He will need to be able to make the patterns he designs.
I tried to teach my daughter because I wanted her to learn to sew, the only problem was she had "zero" interest, so it was not a very good experience. I have no ideas to offer you, but I wish well on your new adventure with him, and I am sure you both will enjoy the time together learning this new hobby for him.