There was much resistance to grade skipping, acceleration etc from my son when he was a kindergartener and all the way up to 4th grade. His anxiety and outright refusal came from worries that his current classmates might forget him and that he wanted to continue meeting those kids outside school for playdates and concerns that he might lose the camaraderie he experienced, sitting with familiar faces all day long. Another reason was that he was already in the 1st grade classroom for core subjects and he sensed that skipping a grade or two in that school would not meet his needs.
We changed schools (for many reasons, the skip being one of them) and I took him to shadow other schools and he saw that there were other kids who liked the same things as him and maybe he might fit well. He was also fascinated by the curriculum discussions that he sat through with admissions directors when they explained what their education philosophy was. We told him that no matter what, we, as parents, would ensure that he got the education that he needed even if he stayed at his current school with his friends and we had to afterschool to meet his needs.
He decided that a skip was OK if he moved schools. Now, he wonders why he was so resistant to the change as he is well adjusted and happy. Most young kids think the world of their classrooms and their peers (as they should) and any change to that setting tends to make them uncomfortable. But, if they acclimate to the new surroundings and feel that they can fit in, they are comfortable with the change.
Also, around 4th grade, self-awareness and maturity kicked in and he told me that he wanted more challenges and would not hesitate at a grade skip any longer if it was brought up to him.
My suggestion would be that you arrange for shadowing the 2nd grade class for a day or two so that he is able to see for himself what the content is, how the teachers teach the subjects and interact with students etc. That would give your son something to look forward to when he evaluates his 2nd grade year.