An idea that popped into my mind.
I just had a visual of taking a sturdy pair of gloves - sew about a foot of so of elastic (do this on each glove)- like for waistbands so it is sturdy but nice and springy - sew the one end firmly to the palm of the glove. Sew the other end firmly to a tennis ball or something not too damaging but not too light - it has to have a bit of weight. He might really like the dangly weight - he can whap, whap, whap things with the balls - including whapping the two balls that are sewn to the gloves together. I can just feel it as I am thinking about it and I would be fascinated. If he likes them, it is pretty hard to pick up other stuff and they might keep him plenty interested. It just seems like a cheap, simple thing that might work. I would have so much fun with those - stims the limit - swirl them, bop them, dangle them, watch them go back and forth like a pendulum.
Just thinking. LM
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Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein.