I think its totally normal that you got turned on by the site of a very large penis, you're only human. It doesn't matter who the dick belongs to, its a huge cock, you can't help what feelings you have. But like everyone else has said there is a line you have to draw; you can't control how you feel about his cock, but you CAN CONTROL what actions you take. Persuing it would be ahuge blow to your son's psychological state and the ramifications of incest are incredibly unpleasant for both of you. Don't feel guilty if you start to fantasize about his cock, or find yourself wondering what it would be like to have an experience with it, feel good that you can control yourself not to let it happen. Feel good about your actions, not your feelings. YOU ARE IN CONTROL OF YOUR ACTIONS.