Hi, I potty trained my oldest son at 2 1/2 also, he's now 5. Here's my advice:
Ask your son every half hour if he needs to go potty, and if he tells you no, tell him you'd like him to try anyways. Take him into the bathroom and stand with him while he tries. If he's not interested you could try something fun, like float some cheerios in the bowl and have him try to sink them. This only works if he's standing. My mistake with my son was assuming that sitting would be easier for starting off, but he wanted to stand like his dad, let your son pick his method. If he does go, you can wait for an hour before asking him again. Also when he does go, reward him with something he loves. With my son it was these fruit gel sticks made from real fruit juice that he adored and we made them his special potty treat. If your son loves fruit snacks (I always recommend the ones made from real fruit juice, but pick something he already loves) then you can use those. The trick here is to make his favorite treat a "potty only treat". Once I wanted to potty train my son I made it clear that he had to go peepee in the potty to get his favorite treat. He wouldn't get it at any other time. He was potty trained in two days. You just really have to stay on top of it and be consistent with asking him if he needs to go potty and having him try anyways. This helps teach him what it feels like when he needs to go potty and it also teaches him to pay attention to his body.
The most important thing you can do is stay calm and not make a huge deal out of it. Kids can be notoriously gun shy and if you make too big a deal out of it, he could freeze. Just keep your cool and be consistant and he'll get the hang of it.
Hang in there and remember no kid ever went to college in diapers. :)