He learnt to 'walk' on his knees when he was about 12 m.o. - and seems unmotivated to walk on his feet. I've seen him do a couple of steps unsupported, he'll potter around with a push-along walker and no problems with standing unaided.
I've been pretty relaxed about it. Walking on his knees has always seemed quite sensible way to 'walk' - he spends a lot of time with boisterous older kids, and knees are obviously harder to get knocked over from. Getting a little fed up now he's 17 months. Would really like him to able to join in running around when we go to the park.
So far I've provided push-along toys and low furniture to cruise around - and just waited for him to figure it out himself. My dad had good success with 'forcing the issue' when we visited for the weekend last month. Every time Ds went to his knees, my dad would hoik him to his feet. Ds kicked up huge stink, shot my dad some filthy looks, but did actually 'perform' once he'd got over being annoyed.