Funny - my DD10 still descends stairs this way, and I can't figure out why. She does other things well (riding a bike, skating) but this has always puzzled me. She _can_ use alternate legs but seems to prefer going one step at a time.

OTOH, my DS7 (who may have DCD) does stairs just fine. Although he does share some other similar traits to the OP's DS (hates to do arts and crafts, doesn't really do Lego).