Because he is feeling overwhelmed with this new responsibility.
He is a child that is used to being able to do things the first time and then always be able to do it after that..
He is feeling everything at once.. Stress, shame, frustration and nervous.
He wants to succeed everyt ime. He does not want to disappoint you because YOU re so excited when he does it correctly. HE is freaked out,.,
He is not even 2, so also his verbal skills are not matching up with his feelings yet..
I suggest you not be so excited when he is successful and to actually explain that it is ok if he has accidents. That you are not mad. That you JUST want him to try..
Also you and your husband need to model making mistakes and how to respond. Support each other. If you drop a cup, react verbally,
"Uh oh, I dropped my cup. I need help cleaning up."
"Oops, I forgot to by apples at the grocery store, oh well, I will eat and orange instead.. "
"Oh dear, you had an accident, ok, lets get you changes."
"Wow, you made it to the potty, I bet you feel better."
I never rewarded our daughter when she was successful at anything that was just expected behaviors.
I always just told her to do her best..
If it did not turn out well, I would ask her "did you do your best?"
If she said "yes", I told her, "well that is all that I ask". If she said "no", I asked, "what do think you will do next time?" Or "how can you do it differently next time?" " Do you need help?" "How can I help you?"
"I like that at least you tried."
Just pick one for each event..
It is still early in the training.. it could take weeks..
I am sending you strength