I admire the fact that he's confident enough in himself that he doesn't care what people think about him

This isn't confidence, this is sheer childishness and it is not worthy of respect.

Part of growing up is accepting your place in a larger social unit, whether it be your family, your group of friends, your workplace, or even larger abstractions like a country or religion. An adult places a value on the happiness and wellbeing of others, and although it is proper for an adult to put his own needs first, a mature person will accept small inconveniences out of respect for the greater whole. We stop and give directions to someone who is lost, even though it delays us for no benefit. We soften criticism toward our friends and loved ones to avoid hurting them. We tip waiters even though we technically don't have to. We do activities we don't really enjoy to support the interests of our friends and family.

A mature person can evaluate the difference between those social norms which are actually harmful and worth resisting, and which ones exist because the world is a nicer, friendlier place when everyone agrees to follow them.

Your boyfriend cares so little about your friends' marriage and your grandfather's death that he would rather publicly disrupt their ceremonies and make his girlfriend uncomfortable than change his clothes to something socially normal. It's not a question of whether he values the solemness of funerals in general, he is knowingly behaving in a way that he knows will be interpreted as disrepectful towards the people who might be his future family.

Your boyfriend has serious narcissism and maturity issues. He doesn't place any value on the happiness of people who are not himself, including you. This is a much larger problem than just this funeral, and you will need to address his attitude if you want to stay with him.