How do I politely and tactfully tell my advisor that I would like more consistency in meetings?
Tell him you would like to have more guidance, and ask to have regular meetings with a fixed schedule, same time every week. This way he will have a permanent entry on his calendar, and be more likely to reschedule tasks around it.

Some weeks you'll have more results, some others you'll have less, but it will be good if you can keep having at least a short chat every week.