I don't think low humidity alone is to blame for the snake not eating.

Your low humidity problem is probably because of the normal weather in your area, maybe when you set up the tank, the humidity was higher. Now that it is winter, the humidity is basically nothing. The cypress mulch should help along with an occasional misting.

Part of the reason why he might be off food is because it is breeding season and he is of breeding age. As long as he isn't loosing hundreds and hundreds of grams, its nothing too much to worry about.

You could try increasing his temps or lowering his temps by a few degrees and see if that tricks him into thinking the season is over.