by Aratus » Mon Aug 14, 2017 10:50 pm
It is always worth the additional cost of greater aperture. It isn't just an inch difference, it is surface area. Remember the old 'Pi R squared? 5 inches is 19.6 square inches. 6 inches is 28.3 square inches. That's a 30% increase in the light coming into the telescope. In the end it is a question of the money which only the owner of the money can answer.
If your son is serious about the hobby, (it sounds like he is) and you can find the extra money, then the 6" telescope will be a fantastic telescope that will provide access to a great number of objects in the sky. Having said that, a 5" is a fine telescope too, but the 6" will bring in more.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.