This is an article about responsibility on many fronts. My opinion (for what it's worth):

Your son has no responsibility to attend school, so missing it is a non-issue. That false "obligation" was thrust upon him without his consent. He has a natural responsibility to learn and grow, but that has nothing to do with attending an institution. If he wanted a note, I'd have given it, if he didn't care, neither would I.

The most important point is the misdirected outlet for his budding social interest. It is exceedingly admirable that he cares enough to protest, and equally so that you placed responsibility for the environment squarely upon him. My reply to his actions would be:

"Why on Earth would you go beg politicians to do anything? They are not there to help anyone but themselves. They're largely criminals who use and abuse others for personal gain, and if they will do this to people, why do you think they'd care about the planet? Don't look to them - it's not their responsibility to act on anyone's behalf - and certainly don't trust in them. Make the necessary changes yourself, and encourage others to do the same."

You gave him the last part, but the first part is extremely important too.