Bernie was also booed at I heard.
He was not booed at. His supporters at the DNC cheered him for four minutes straight. It is when Bernie said that the party needs to unite behind Hillary, he got booed directed at that statement and towards Hillary. He was honorable and still supported her, but he surely feels stepped on by the Democratic party. He was right, it was rigged.
He did a good speech for his followers. Tons of support for him, and protestors outside in mass supporting him and chanting, "Hell no Hillary," or something like that.