If you can afford it get him a fretless electic. There are less expensive alternatives to the Fender series basses. Maybe start there? Do an internet search for 'fretless electric bass'.

There's an effect called a 'noisegate' that's largely used on drum mics to keep them from feeding back in volume controlled situations. The noisegate allows the mic to open when the drum is struck and holds open for a certain period of time.

Have never heard of a pedal noisegate effect, but I imagine there's probably something similar available to control sustain. Either built into the amp or an inline effect.

Hopefully others w/a more refined knowledge will chime in soon. :-)