The Ibasso D4 accepts a USB input and the Ibasso D10 will accept USB and optical input. Both can be powered by the USB output of the source.
Last night I paid $250 for a used Ibasso D10.
I am not sure if my next idea will interest you but maybe custom-fitted IEMs would be a cool surprise. I went overboard and purchased JH13 PRO (
www.jhaudio.com) IEMs but there are more cost-friendly options and different suppliers. I wanted to throw this idea out there because custom-fitted IEMs have wonderful isolation and many are easy loads for a laptop, iPod, and cellphones. Instead of a DAC/AMP combination a custom set of IEMs could be the ticket.
Sorry to keep going on. There are also USB soundcards or DACs alone that can be found cheap that may drive the Grados effectively enough depending on your son's listening habits.