Help please

Potty training disaster! I need advice please! 
My son is 2 and I know he's not ready to be fully potty trained. But my mother has been insistant in potty training him since before his birthday. I told her no many many times and every time he'd stay the night with her she'd try anyways. So I was weak and I gave in and they said for them he's been doing awesome, but here's were it gets scary. Last night I Tryed to get him to go I put him in his undies and he threw a small fit so I Tryed explaining to him that it's ok this is what big boys do so 30 minutes later I asked if he needed to potty he said no so I waited 10 more and said lets go try and that's when the hysteria starts. He fights and cryd like he's been beat just to take them off and then to sit on the potty he acted like he was terrified and then he wouldn't get off the potty and refused to let us put his undies back on it was the worst fight he'd ever thrown he acted scared to death my son is very happy go lucky and is usually very brave and boy like to say the least this was not him at all I've never seen him so worked up and scared! What the heck is goin on?!? What should I do? How can I reverse whatever she's done to him?