Here's a link to a PDF showing Trail Boss info. Trail Boss is simple, fill the case and shoot, no room for an over charge.
Depending on the shooter's size and experience at 8yrs old, some of the reduced 308 loads recommended above may be a bit much initially.
Just my opinion, but a 223 or 243 would be a better choice than a 308. I started out with a 22-250 and think it a great choice as well.
http://www.hodgdon.com/PDF/Trail%20Boss%20Reduced%20Loads%20R&P.pdf
