An easy and cheap solution may be just drilling out the butt cap on a currently built rod. You would then be able to insert your rod extension into this, thus making it longer. For the extension I would cut the butt section off of a rod blank that will snuggly fit into the rod, doesn’t have to be a fly rod. You could build a sweet custom grip that could help lock the rod into his arm pit and then use what's left of his arm to steer the rod.
This could be a good first option, as you could convert a decent used rod with out spending money on a custom build until you find out if he likes fly fishing.
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