Normally, what time would he poop? Predict it, and take him to the potty pad before hand. I've also seen with my dog that walks or activity help stimulate the need to go #2. This is going to sound gross, but have you tried rubbing his poop on a fresh pad so he can smell it? Maybe he associates the pad with pee and not poop.
__________________ Littlest JakJak 
We miss you Kaji 
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