Mathew is correct about the price of ARFs vs kits...

BUT I have seen a lot of people get into the hobby with ARFs (Almost Ready to Fly) and then quickly drop out because they lacked the skill to repair the Air Craft or fully understand the workings of it... [] [:'(]

Another "Theory" might be that those who purchase their first AC as an ARF lack the commitment to the hobby... either way I would recommend a good kit to get you and your son into the 'Full' modelling mind set.

Building the AC together will also be a great Father and Son experiance - even if it is not as 'Economically Viable' as an ARF... it fills the evenings and will build a bond between you as well as the appreciation of creating something together. A tad poetic perhaps... bu t nontheless the point is there.

Have a look about for the "Eagle 2", "Serioretta" or "Kadet", they are all excellent trainers and lots of fun to build.

Good Luck!!