The kid doesn't need to be a muslim to be a terrorist. He had to know that people would suspect terrorism if he blew himself up. Blowing himself up, knowing that people will suspect terrorism kinda make him a terrorist in his own right.
For all we know he could have fully intended to detonate in the crowd as they exited the stadium but got fidgety and POOF.
For all we know he may have wanted to cause an explosion near the KU buses as a prank. From what I read he exploded near where the KU buses were parked. It could be that he was one of those weird kids I knew in school that thought it would be great fun to blow up the other teams buses as a prank. Alcohol and dynamite dont mix.