Best one I have seen up to this point to assist with playing to where he doesn't need much strength in his LH to hold it:
https://www.musicarts.com/tuba-essentials-the-hug-adjustable-euphonium-stand-for-right-side-mouthpiece-instruments-main0461634

Regardless of the equipment you get for this student, you should NOT be having to pay for it yourself out of the band budget. Check with their 504 campus coordinator to make sure that this accommodation meets their needs and can be provided by the school/district for them to perform in the classroom with their peers.

Thank YOU for including this child in your music making! :-)