Hi,
I'm going to assume from your post that your son is going to start Year 4 in September (which is classified as Under 9 by age groups in all sports). There are only a handful of Counties that run Under 9 development squads, but virtually all include Under 10s as part of their development programme - and these squads include U9 and younger players if they're good enough. Saying that, I am not sure that Gloucestershire do, as we (Oxfordshire) do not play them. It is not unusual for an eight-year-old to play in U11 sides: representative selection/trials will be looking for more than this: all-round players who can contribute in both departments, or outstanding fielders, will be high on the priority list.
Qualification to play for a County youth side is based on three things: (a) where you live, (b) where you go to school, and (c) where your club side is. So, based on, this, I'd suggest you contact the Cricket Development Manager for the relevant County, and ask them how best to proceed.
Gloucestershire's CDM is Steve Silk - email Steve.Silk @ glosccc.co.uk
Somerset's CDM is Andrew Moulding - email andrew.moulding @ ecb.co.uk
Best of luck, and let us know how you get on!