T.T.
My now five year old started out the same way at about the same age as your son (he was 27 months). He wanted to go in the potty but didn't always know when it was time to go. We recruited his beloved bear to help with timeing until he could recognize the sensation of having to go on his own. I put Christian in underwear at this time because he was only going in his pull ups for convenience.
I told my son he should show bear how to go potty. I got bear out of his room every hour and a half or so and put him on the bathroom counter (when Christian wasn't looking). I told Christian to go see if bear thought it was time to go. He went into the bathroom and yelled "mommy he's here!" I'd say "ok you show bear how to do it". Most of the time he used the potty, if he said he didn't have to I told him to show bear how to try. Afterwards , we gave him a ton of praise. When he finished it was bears turn (Christian decided this). We put bear on the floor in front of the potty, stepped out and shut the door for a second. We went back in and gave bear the same praise. Together we returned bear to his bedroom until the next time. It helped that bear went last so Christian couldn't opt out. If Christian didn't go or try, bear wouldn't go or try. By 30 months old Christian started realizing when it was time to go and would dart off for the bathroom by himself.
For us I think bear filled in the time gap between Christian knowing how and wanting to use the potty until he could actually feel the urge to go.
While useing bear as a "timer" Christian still needed night time pull ups until he could feel the urge to go.
I hope this helps.