I would as quickly as possible remove him from the situation. So get him dressed quickly and put him down and say "No kicking mummy" then leave the room. Then when he doesn't kick (this is more important and effective than saying No IMO although I would still do both) PRAISE when he doesn't kick, if he's distracted or whatever doesn't matter praise him and cuddle and kiss when he doesn't kick and say thank you etc. I know it sounds extreme but the only way I have ever been able to overcome unfavourable behaviour has been through praise. I still think its important to say No so that they know what they are doing is not ok. But the praise shows them how wonderful it feels to do the right thing.

Hope I'm making sense.

*hugs*
Cailin