have you tried showing him? (and wait til he has tantrums - he'll start kicking you then!)
maybe putting a ball in front of him when he is walking to encourage him to move it - may then progress to kicking? I don't think its anything to be concerned about though - maybe he's just not interested yet?
mine was the same too - kicking as soon as he could stand up, but my dh plays football, so was maybe influenced by this? - how is his balance? if you are worried though you could always mention it to your HV?
ds has kicked a ball from 13 months: we didn't try to teach him but he copied older children in the playground. however he has always been extremely interested in footballs , i think this is because we have a dog who chases them obssessively. does your ds see other kids do it? and is he interested in playing with balls otherwise (picking up, throwing, etc?) if he isn't much into balls, he probably doesnt see much point in kicking...
Coincidentally, my mother has just told me that my nephew has finally started kicking a ball. He is 3 years old has has a football mad father who was starting to get very worried by his son's lack of interest.