We did a pull up over underwear the first time we went in the car. Didn't do a puppy pad cos you shouldn't add anything to the car seat for safety purposes. Send him to nursery and tell them how you are approaching it.
We followed the method in the ‘Oh Crap’ book. Highly recommend it having a read, I wouldn’t be surprised if they had a chapter on travel / car etc. but it worked a treat. Had almost 3 days at home , offered lots of juice which they’re obviously happy to drink but gives them lots of practice as they’ll need to go quite often. It might also increase the accidents initially but they quickly learn to sit on the potty. We initially had the potty in the room with us and then moved it slowly to the toilet as she became more confident controlling her bladder. It worked really well for us, every toddler is different but it’s been a hit amongst my friends anyway :)
I’m in the same boat. We’re two weeks in and he’s now only just starting to take himself to the potty without me prompting. I just remind him that the potty is there occasionally. First time we tried nursery we’re really keen to carry on but we both weren’t ready. He went on Thursday and they ended up putting him in a nappy. Devastated. Thankfully he’s come back, having 2 max accidents a day while he’s learning to tell us. I used a puppy pad in the car, bring the potty too. Speak to nursery and make sure you are all on the same page. We have nursery Wednesday and may say to put a pull up on but I really don’t want him going backwards.