It's pretty obvious when one gets stonewalled for political reasons when attempting to contribute to a project.

He doesn't need to provide specific examples or explanations, doing so just makes the issue worse. Often these issues involve specific people and specific intuitions that will be hard to interpret without lots of context. It will put the problem people on the defensive and likely cause some sort of outrage or personal attacks, or other unwanted drama.

Being polite yet firm on his own page is the best that can really be done in these situations. It allows the technical problem to be magnified while not focusing on drama.