We've always told Jacob 'no' or 'uh uh' whenever he does something like that, e.g. "We don't hit Jacob, hitting hurts". Then we would try to distract him with something else immediately (at about Kynan's age - distraction doesn't always work so well these days LOL). Can you give him a special 'nappy change' toy or book - something that he only gets with nappy changes, that will distract him perhaps?

Like Cailin said, we use a lot of praise here too. So when Jacob does the right things, we praise him for that and tell him how good/helpful etc he has been. Even just playing quietly on his own, we praise him for and tell him how well he is playing, or how great he's been whilst Mummy was on the phone etc. So the times when Kynan doesn't kick during a nappy change, tell him how good he's been and give him lots of cuddles/kisses etc.