If he's playing defensively and waiting for you to approach him and make a mistake, either wait for him to come to you or trick him into making a mistake. One of the more basic examples of this would be to run in then wavedash back to bait him into throwing out an attack, grab, roll or anything else he might do. Then punish with a grab, smash, or whatever fits the situation.
If you both end up not approaching each other for a while and he gets impatient and tries to come in for an attack or bait you into attack when he comes in, then it's your turn to punish a mistake he makes or make him make a mistake. Maybe he has bad spacing and he hits your shield to far in, then shield grab him. Maybe he runs then stops or wave dashes back trying to make you attack, grab him. Tons of other situations, but I was just giving some very basic examples that might get you thinking.