My first piece of advice, NEVER ASK him if he has to go, just every hour and a half to two hours put him on the potty, no discussion, if he goes make a huge deal out of it, if he doesn't then it is great too. Just say you need to try to go. Do not make it stressful as he will rebel and it will take a lot longer. 2 1/2 is young for a boy, which typically take longer. I started with my son at about this age and he didn't have any interest. However by his third bday he was in big boy underwear all day, two mos after that he was 100% potty trained, pooping, peeing and at night was in underwear. Just be patient, be matter of fact about it and take him to pick out his own cool underwear, that was the deciding factor for my son. I am a firm believer until they are ready to use pullups despite all that say otherwise. Number one it lessens your stress, you aren't cleaning pee and poop up out of underwear and it still gives them a sense of being a big kid.
Worked with both of my kids using pullups first. Just don't ask him, tell him he "has to try for mommy or daddy" and set him on it. Explain if he doesn't have to then that is fine, but let's try. Not a choice for him. This will help a lot. boys get distracted and easily would rather mess their pants then stop playing so don't wait for him to tell you. Good luck, just keep at it...it will click soon enough.