Hello Edgeware,
My DS8 started knitting at that age. I bought him children’s knitting needles, not jumbo but shorter length. I offered a choice of carefully chosen DK yarns (from my stash of leftovers), ones that don’t twist or split and are not too slippery, that I’d used before and knew were easy to work with. I cast on and off for him at that age. My DS’s first project was to knit a garter stitch scarf for his teddy. It was a case of very close supervision but some 6 year old’s can have the dexterity for it. Initially literally 5-10 minutes at a time and when the child has got the hang of it you can build up to longer sessions if desired.
If your son really is too young, would he give sewing a try instead? It offers the same benefits, keeping hands busy, but it’s easier. Amazon has nice tapestry kits for young kids, the canvas has a low hpi so each hole is large and easy to see, the tapestry needle is blunt. My son’s school introduced this type of sewing in Year 2.
Re knitting supplies, children’s needles are approx £3, available in high street knitting shops and maybe The Range; most charity shops sell new yarn, people donate the 1 ball leftover at the end of a project. IMO jumbo needles look awkward to work with. Buying needles and yarn is cheaper and better than buying a kit. OTOH, I like the little kits for sewing (unless your child has a design / image he wants to create).
I’ve attached 2 photos of the sort of thing I’d consider suitable at the 6-8 age range. Feel free to ask any further questions or PM me. It’s a lovely hobby to keep children entertained in wet weather 😀