You did the absolute best thing. You gave him a chance. Bystander CPR is sometimes successful. His chance was likely very small, but if you had not jumped into action, his chance would have been 0.
You should be very proud of yourself, as many people would not have felt able to do this, so please be kind to yourself.
Do not wonder if he would have survived if you had only done more. There is nothing else you could have done. Even in hospital, where we have all the kit and all the team, and the ability to rapidly test for any reversible cause, less than 25% will survive to being discharged home.
The very thought that you tried to help and that somebody cared will be comfort to his family.
Some years back, my friend was killed in a RTA. He was conscious but obviously dying at the scene. A bystander stopped and prayed with him. I don’t know how much he was aware of that, but he had a very strong personal faith. I take comfort in the fact that someone gave that care to a stranger, that he didn’t die ‘alone’ and that he had some sort of last rites, which would have been important to him.