If you only ride twice a month, I don't see how you can accomplish your goal. If that is all you are going to ride, perhaps a bike with an electric assist motor would be good? Otherwise, you are going to be one mighty sore and tired person after that 25 miles, IMHO. Most of us ride several times weekly, and several hundreds of mile per month.
You really need to set yourself up a training/riding schedule and build into the ride with your son.