When do be concerned that my son has no interest in walking whatsoever?

My son just turned 13 months old and really shown no interest in walking what so ever. He is a super fast crawler and cruises effectiently. He can get on and off furniture safely and climbs full flights of stairs. He just shows no interest in actually letting go of what ever he's holding on to for balance and try to walk on his own. He doesn't stand on his own at all. He does love pushing things with wheels like baby walkers and stuff like that, but won't walk with me if I hold his hands. He is developing normally in every other way, but just has no desire to walk or stand on his own. When do I start being concerned about this?