My son appreciates warnings. If he won't go before we leave the house I tell him when we get to where we are going we are going to the potty first. It will get easier, but you just have to find a rhythm. If you know he goes every 2 hours just make sure you plan accordingly, and tell him the plan and your expectations. If my son obviously has to go but won't stop playing I say "I can see you have to go, it's time to stop what you are doing and go" there is no option or question in it.
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