Here's how you encourage him, and please listen to me on this, because I know what I'm talking about:
STOP trying to get him to read to you. YOU just keep reading the books to HIM. If you make this process torturous, he's going to start to hate reading, and then you will really have a problem. If you create a warm, fuzzy, magical feeling around reading (snuggling up with Mommy while she reads to him), then he is going to have a positive feeling about the subject of reading. If the subject of reading evokes emotions of battling with Mommy, then he will dislike it.
He's only 5, so you really don't have to worry about him being harmed in first grade. Remember, teaching kids is what the teacher is for. Also remember, "Education is not the filling of a bucket, but the lighting of a fire."
Don't squelch your son's fire by making reading a chore. Right now, he just wants mommy to read to him. Have fun reading to him.
And if, when he's older, he really still seems to have trouble reading, you can have him tested for dyslexia. But I doubt that's it.