I cant help with the potty pad training... but what I did with Gracie is if she had an accident I would put her in "time out" and take away all of her toys until she went potty outside... or in your case on the potty pad... but has he gone on it before? because maybe he is confused?
maybe he doesnt know where he should go??? I hope it helps... and I didnt "punish" Gracie until she was a little older... but Ive only had to do that 2 times and she hasnt had an accident since... so good luck!