How to potty train a toddler?

My son is 2.5 years old and is eager to potty train. He asks to go potty all on his own. But he will only use a small floor potty. He is absolutely terrified of the big potty. Even with a step stool and toilet seat for him. He will scream and cry and start shaking if you try to sit him on it. I have tried sitting him facing backwards, and holding him while he’s on it for comfort. I don’t know how to get him to use the big potty. Any advice would be VERY appreciated!!