The ultra is a cracking rifle but is difficult for a little 'un to cock. The s200 is still quite heavy and will be too long for an 8 year old. the Brocock again, although compact and light, still has a stock designed for adult reach. The only real option is to buy something second hand and find someone who can chop a section out of the stock. If it's done right then the section can be added again later if selling or if your son grows. I had a BSA ultra for the combo fund that had a section chopped from the stock and that made the rifle a perfect fit for the 10 year old it went to.
oh, the ratcatcher is also designed with an adult reach.
Actually just remembered, i have a BSA ultra stock that Kieran T stripped for me. It's stood in the garage gathering dust. If you go for an ultra then let me know and i'll get someone to take a section out of the stock and refit the but pad, you can have it for nowt
Edit, best advice is look for a local club and take him there.