You both need to be ready. If it's stressing you out and you're passing that stress onto him, he's going to refuse. If he gets the idea that you 'need' him to do it he'll most likely rebel. It can be a boiling hot day & if I ask DD to take her jacket off, or put a t-shirt on, you can guarantee the answer will be no!
Set aside some time when you feel 100% ready to tackle it in all its glory! For me I cleared the schedule for a week or so and committed to no nappies. Once upon a time that thought terrified me! But I don't really know why. It's just wee. A quick change & it's all over. For 2 days DD had nothing but accidents. The only wees in the potty were flukes. Day 3 she woke up and said, "mummy a wee is coming!" And that was it. Fully trained day and night ever since. She was so resistant for so long but I think it was my commitment and relaxed attitude that helped her through.
Good luck. He will get there, they all do!!