Potty training 3 year old help

My son will be 3 next month. He HATES potty training, to the point of holding his urine. We tried a few months ago and he did great on day 1, then be totally regressed and began holding his urine. He hates being naked. He is okay with sitting on the toilet, but refuses to let it go. He will hold it all day and I know it’s so dangerous for a toddler to hold their urine for that long. The first time he held it, he was wincing in pain. Since then, we have tried a few more times and he still holds it. Any suggestions? We have tried rewards, positive praise, a lax approach. He is a skeptical child by nature, so I’m not surprised this is his approach, I just want him to get over this difficult hump. It makes me so sad.