Potty Training Help Please!

My son is 3.5 years old and we are struggling with potty training. We've done the naked days, incentive treats, taking him when we go, all of that. The problem is that he holds it until he gets a pull-up on. He's seeing a neurologist because we do believe that he's on the spectrum to some degree but not significantly. 
We had 4 hours naked, I gave him tons to drink and he had tiny little dribble accidents but would pee in the potty but as soon as I put a pull up on (because it was time to start dinner) it was full to the point of bursting and needed changing almost immediately. He's always been this way too ever since he was a baby, he won't pee or poop in the bath and would want out so he could go in a diaper. I'm at a loss on how to overcome this mental block. 

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