Welcome to YorkieTalk.
My younger boy is a "runner" from his own poop when he is outside. I think it may be an instinct, wanting to get away from the thing that would attract a predator to his trail.
If your boy isn't shy, try standing near the pad when he is doing his business and when you sense he wants to run, hold him on the pad and with a reassuring voice tell him to stay. When he finishes, let him go and praise him profusely and reward him with a treat. Use keywords and phrases to tell him to stay and finish.
Make sure the area around his anus is kept trimmed so that the poop has a clear, easy path to the pad.
It may take time and a lot of repetition, but he will eventually get it.