Help!!!! my dog ate a big slice of cheesecake and I don't know what its going to do to him.?

I left some cheesecake on the table in my dining room and my 6 year old dog Chi-Chi got into it. When I came back in the room it was all on his nose and mouth. I hope he doesn't get sick. What should I do he is acting like he is really tired or lethargic or something.