Hi Gretchen,
check out the Ultimate Switch on Enabling Devices website (65.00) enablingdevices.com/catalog It comes with a goose neck mount so it is easy to place the switch anywhere you want. I have mostly used it for kids that are using their head as an access point, but I think it might work placed properly in the line of movement as he flexes. SETC has an Ultimate Switch you can borrow. Check out our website.
www.cwu.edu/~setc/Kristin