I would have to say whatever your son usually wears and is comfy in is best to bring - maybe sandels and sneakers as well. Like others have said break them in at home and make sure they are comfy, nothing worse than new shoes at Disney.
OT a bit, but don't completely rule out a stroller, at 6 my dd needed one for those long evenings like fireworks nights. Epcot WS was our favorite evening destination and by 5ish she was to tired to walk but could still very much enjoy the evening in a stroller. We would always start out the day walking esp at MK and MGM but by late afternoon or evening dd always started to fade and a rental stroller was well worth the $$ IMHO.
WDW is alot of walking and it kept our dd's happy for the trip. By 7-8ish they didn't need one even at night so you may be ok if you take a mid day rest - but again don't rule it out completely!!
Have Fun
TJ