Its up to your trust level and risk tolerance but biggest things I can see is its your money/*** on the line. Some stuff they will do won't be worth 50% of the deal and other people do not value your money...they will spend your money like water on a materials/tools they wouldn't spend if they were financing it, ect
I would recommend being outside of the deal so to speak, but with a close eye on it, similar to a banks perspective:
(Look into 'Hard Money Lending')
Your son & GC could run their own flip or rental business. Flips - They invest certain %$ down so they have skin in the game or what you're comfy with and then pay you a good interest rate of 6-20% for a certain lenght of time 1-6 Months. You have to be paid off when deal is complete or their able to refinance with bank (which is tuff so that is why your success is determined on the 'Buy' end of the deal) If its a successful flip, they make money and so do you. If it fails, they lost money, you lost money but you'll atleast have the actual asset to take over and hopefully recover losses.
If their rental properties, you carry the note.
All in all, either route, you want to be involved to make sure YOUR money is invested into something that is worth it, but not too involved that you get the shaft.
PS: I'm in a smaller town and have been buying pretty cheap as well.