My boyfriend is so impressionable and I hate it!! What can I do?

My boyfriend is 23, but he is so easily impressionable that you would be surprised he's 23. During the summer he joined a union and it seems like ever since then, he was brainwashed!!! Today was Canada's voting day, and he changed the party he usually votes for because of the unions beliefs. He is trying to get me to change my voting opinion as well, and it really bothers me. He is so easily brainwashed and influenced, and this is one example. I want to tell him but I know he'll get really offended. What can I do?