Hi N.,
It's a good sign that he tells you when he pees, even though it's too late for the potty at that point. You should praise him for letting you know.
I'm a 27-year-old mom of a two year old son. He will be three in February and is not yet very interested in using the potty. At daycare he will pee in the potty for them, but at home he rarely will. And just like your son, he will often tell me has gone potty in his pullup after the fact. I talked with my doctor and they said many children, especially boys, aren't potty trained until well after they turn 3. She said not to push them if they don't want to go...you can only ask, but if they say "no", they're not ready. It's important to remember that there is a physical factor behind potty training as well....their bodies have to physically have the ability to hold their bladder when appropriate and some boys don't develop that ability until after the age of 3.
What I've done is have my son try to pee right before he gets a bath. At that point he's naked and I just sit him down on his potty. Most times he will pee. I praise him, and then hope that he will eventually make the connection that he can pee on the potty at any time throughout the day and get the praise he loves. If I get him potty trained soon, I will let you know what worked for me. Good luck!