M.,
My son was just potty trained about 6 weeks ago and it's worked very well; he's pratically dry at night now! so he's really taken it to heart. However, he was nearing 3 and a half when we began. I sure know about the standing there watching helplessly while you watch your kid go in his Pampers, but the good news is that the way he rejects your proposition is in a knowing way. He knows now that he's doing it in his Pampers when he could be doing it elsewhere.
What we did was this: as we both work full-time with boy in day care, we first waited until we had a 10-day vacation period with him. (That will be different for you as a stay home Mom.) Then, we prepared him for it 5-7 days before the vacation. "When you're on vacation you're going to start doing peepee/caca in the potty." To motivate him, we put up a printout of a large calendar that highlighted those vacation days, and entitled it "Cameron's Points." Each time he did a pee pee it was worth 1 point, a caca 2 points, peepee/caca 3 points. We printed things he liked on a sticky label paper (picture of Neemo, Dora, Boots, Dora & Boots, a lion, a dolphin, etc.) We rewarded him with the stickers placing them on the calendar day when he did good (he got to stick it on himself!) And we could track his progress like this, too. For example, 21st March he got 4 stickers, and by the end of the 10 day period he had as many as 11-13 stickers a day!
During the training we lived normally, and of course, he made mistakes (esp. caca = poo poo) for about the first 5 days. When he did poop in his pants at home he was responsible for walking himself up to the bathtub and taking down his pants (with a little help, of course). The idea is to make him "suffer" the uncomfortableness of "going" in his underware, and make him responsible for "helping" with the cleanup. He once did a caca while playing at IKEA, and marched with legs apart to the bathroom looking rather sheepish. Of course, I helped change him, and NEVER scolded for his errors. On the other hand, he got his points and lots of praise for using the potty correctly.
Naturally, it helps to ask from time to time if he needs to go, as initially they do need reminding. Also, let him sit on the potty with a book or in front of a favorite t.v. show. Plus, it's obvious you need to keep a change of clothing (include shoes and socks) in a backpack, and/or also in the car. And, expect he won't be perfect at first, so bring all your changing stuff (baby wipes, towels, plastic bags for dirty clothes, etc.)
At his age it will come pretty fast, but still, be patient with him. Once it "clicks" in his head, he'll soon be an old pro. Good luck!