Advice and ideas for Potty Training a little Boy

My little boy is currently 2 years 5 months. We have been trying to get him interested in the potty since he turned 2. He has very occasionally sat down on the potty and done a couple of wee’s but generally refuses, says ‘not yet’ or will sit on it and do nothing until he gets a nappy put back on. When he has a nappy off he will hold it in. He 100% understands, he says when he needs a nappy change and was even really cheeky and said he would finally sit on the potty when it was bed time the other day just so he could stay up later! Just struggling to get his opinion to change and wanting to do it. He spends 2 days with Grandparents and 2 days at nursery when I’m working. Both grandparents do the same and put a potty out and encourage him & I have asked nursery whether they can even just take him when others are going/ potty training in the hope he will think it's 'cool'. Any advice & idea’s welcome… He picked out ‘Big boy pant’s’ but he won’t even put them on top of a pull up never mind wear them. I wondered about a reward chart but I don’t know whether he would get that at the end he would get something from it? We have also tried the actual toilet and know’s we go on that but won’t go on (he has once in the past). Parents have said that we all were potty trained by 2 so I’m wondering if I have left it too late until he is now too aware of everything 🫣. He can be really stubborn. He’s quite a sensitive/ worried little boy so I really don’t want to stress him out and overwhelm him either.