Oh this sounds just like my DD!!! We have gone back to nappies for the moment, she doesn't even want to wear underpants and I was so sick of going through 6 or more pairs of pants/undies a day! Funnily enough though I send her to daycare in underpants and she might only wet once or twice (the Monday just gone she had no wee accidents..). So I know she can do it, she is just very strong willed and if she doesn't want to, it won't happen. Im trying not to force it, not even mentioning her using the loo and she has voluntarily sat on it and done a couple of wees in the last week.
2.5 is still quite young to be toilet trained, especially for boys, so try not to worry too much. I know how frustrating it can be! I'd either keep persisting, or give it a break for a little while. Rewards charts can help, but in the long term I don't know how successful they'd be, I haven't tried. As someone else said to me, it's not like he'll still be in nappies or wetting himself at age 10!