There is so much to say here..... but I'll try to keep it simple.
I was a home-ec teacher for over 20 years and I agree about teaching the boys to sew. They are creative, they aren't afraid of the machine and they are always very proud of their projects. And, I was a co-leader for my daughter's and granddaughter's 4-H group.
Do check with the leaders to find out the categories for the 4-H show. You don't want something disqualified because it doesn't fit into one of the categories. But, having said that, most anything sewing could be squeezed into a category for a show..... unless it's only a garment show.
Your idea of showing your son the steps to making his quilt is really the best plan. Most kids learn better by viewing and doing..... we all learn best that way. Having taught many student with autism, I'm aware of the frustrations..... some will get frustrated and want to quit... others want to keep going, forever. You'll have to decide which your son is and adjust to his needs.
My DGD's first 4-H show she wanted to make a quilt..... she loved horses and we found a pattern with large pictures of horses. We fussy cut the horses to 12"x12" blocks and alternated them with a 9-patch coordinating fabrics. The quilt was easy for her to work on (until the quilting.... that was a struggle) and she still has the quilt today. She got a blue ribbon and was so proud. She's married now and has a 4-year old and she's already talking about Nanna teaching her little one to sew. Of course, I'm really excited to do that..... wish she wasn't 12 hours away.