How sweet! And he is absorbing a lot of what you do, just watching. Nice to give them a little project that's real. Have you thought about having him try simple weaving...my daughter loved that; 1st paper strips, then yarn, and later fabric. My sons did the weaving with medium weight string on cardboard, which the teacher had cut with slots.
My daughter was about 3 when I went into my bedroom and realized she had done a little sewing herself on our blanket. She had taken the string off a sleeping bag in my son's room, and laced it round and round through the open weave of the blanket. After that I got her some patterned "sewing cards", which she enjoyed.
She took her first sewing class in 3rd grade weekly for the year. They made PJ's and tote bags, among others. Each day they made them un-thread the machines at the end of class & re-thread at the start next class. The teacher started them off by sewing through paper (no thread) straight then curvy lines, and all kinds of patterns. Next they did the same WITH thread, and finally practiced on fabric. It was a very worthwhile class.