The post is leaning because its rotten, broken or wasn't buried deep enough the first time. Since it's a fence post and not being used to support a structure or a gate, if I were you, I would absolutely
not use concrete. If you use concrete and the post breaks again you will have to dig out the concrete the next time. Given that its adjacent to your driveway its always in danger of being damaged, even if its not you. As long as your hole is deep enough it's not going anywhere unless you back a vehicle into it and all the concrete in the world isn't going to prevent that.
Rather than concrete you can
put a key at the post like this guy did.
Also, make sure you buy pressure treated wood. Good luck.