We are in the same boat... my daughter is also 2 1/2 year old. She shows intrest and will sit on the potty but does not go in it yet. Done the rnu around naked, training pants, underware w/plastic and so on.
After some resreach I have come to the conclusion that my daughter does not know when she needs to go. We have tired the ever 30 mins sit on the potty for almost a month and we are getting no where. So I am taking a break, even thought we are talking about it and she shows signs of intrest she is not ready, she physically can not tell/feel when she needs to pee/poop until she is doing actaully doing it. We also were reaching a frustration point and instead of keep forcing it we are taking a break. If she wants to try sitting on the potty I let her but I am going to wait another month or two before starting up a routine again because I feel she is not 100% ready.
My daughter did not like the plastic over the underwear, she would take off her pants and the plastic and put the pants back on. It might work for some but it irriated my daughter.
We did try the run around the house naked but after a few days of cleaning up mess after mess I stopped doing it... but I have heard it worked for some. Someone said I should make my daughter clean up her mess but that seemed more like a punishment, like oh you made a mess on the floor clean it up. The response I got about that was it is just like having your child clean up their toys but having a child clean up their toys to me is different then them cleaning up their poop and pee.
Over all you have to do what you think will work best for your son. It kind of sounds like you guys need a break if he is starting to 'fight' it or cry it seems that it is frustrating him and that is not going to help.
Since we have not yet been able to potty train take all of this with a grain of salt, hope others have helpful tips or ideas for you.