If he is fairly regular, make sure you set a timer and take him to go try around that time. If nothing else every hour take him in and have him try. Be more aware of where he is and if he is going to sneak off and try and catch him to put him on the potty.
Fighting him won't work. Getting angry won't work. However the best incentive is taking his big boy priviledges away until her re-earns them back. Just be matter of fact. Simply saying "well I thought you were ready, guess not, okay well let me know". He knows the urge enough to sneak off so that isn't the problem, it is about power struggle.
If he has a few things he earned from previously potty training, collect them back and tell them when he is ready he can have them back. The best incentive for my son was Transformers, he wanted one so badly, however told him those were for bigger boys, he was potty trained in two weeks! No joke. He was also 38 mos then too.
He is still young, regression happens so easily. It is a power struggle now and you won't win. My suggestion,take the yuck factor and headache out of it, put him back into pullups and tell him when he is ready to be a big boy to let you know...then let it go. When he does go by himself, then celebrate each time. Be thankful at 2 1/2 he has come this far as in boys that is rare! :)