I found these options available in black.
https://embroideryonline.com/shop?search_query=cut+away
Have you considered a Heat-Away type? I understand that the heat ones vanish but leave the underneath layer to support the stitches.
https://www.shop.dzgns.com/collectio...ite-sew-n-heat
or WSS? You may not want water on the jacket.
Hopefully, another poster will have something in black that works for the design. It sounds like a fun project for your grandson. Good luck!