Another trick or two that might work is:
1. Instead of a light knee to the chest, step firmly into him and throw his balance off.

or

2. When he jumps, step way back so that he cannot make contact, again throwing off his balance.

The key would be to keep him off balance, they don't like to be off balance. Of course, always accompany it with a firm no or down. Always praise him when he does well.