I'm wondering why your sessions are only once a month? Do you feel like that is adequate? It seems like once a week is pretty standard for psychotherapy, especially in the early stages.

Aside from that, I agree with others that you should get to talk about what you want to talk about. Sometimes getting sidetracked is okay and leads to something valuable but not if you leave feeling frustrated. I wouldn't waste time trying to "steer" the conversation, if he goes off topic maybe just say, "I'd like to finish discussing X before we move on to something else." And if it continues to be a problem maybe you could tell him directly that you feel like he's not letting you say your piece.