Typically a homemade cage or indeed a roll cage always sends the Cog higher than not having one. If it's for your son being a young lad he will be playing and taking it extremes at midnight with his mates or by joining a club and playing offroad. Take a look at the minimum spec for comp msa blue book for a design even if you don't want to make it to spec the design is very strong and will protect driver and passenger in the event of an accident. I think when my son is old enough to drive I would not hesitate to fit a cage to his motor. Two reasons , 1 I wouldpinch it for comps and he will be driving trial competitions from the age of 10 upwards. Checkout Alrc trials in Google or click my Anglian link below.
Teaching the youngsters young, how to react when its muddy slipping sideways and keeping in full control without panicking is worth the £20 a year fee and £20 per event fee alone. Knowing the kids can really drive and handle the vehicle whatever the situation offroad would fill me with absute confidence when they get out on the mad roads.
I know a few adults that would benefit from driving these courses and learning to control their vehicle.
J