Depends. If he is a guest preacher (or you are the guest), and will not be under his teaching often, perhaps you can talk to him afterwards or just drop the matter altogether. If you are going to be under this person teaching on a regular basis you may want to take a wait and see approach and determine whether this a repeated pattern. Also, what is the spiritual acumen of the congregation? It's quite possible the church may be well fed (spiritually) and able to handle more meat. Of course, you never do wrong by asking the preacher about his sermon, so long as you have a sincere desire to understand. I am always encouraged when I am asked about my sermons. It proves people are listening.