crf250r rear spring set up

hey, All! My name is Cliff. Bretts dad. Brett is 13, 5"8" 135lbs. He is riding a crf250r with exhaust mods and wheels etc. We got the bike from a race team with someone that weighs 215lbs. I have relieved the front shocks to the very softest settings upper and lower settings. Also i have relieved spring tension on the rear as much as possible. My concern is the rear still being to stiff. With his weight being @135 and he is already clearing 70' table tops and doubles.( loooking at 90 footers) what spring do i need for the rear as a starting point? I believe the front shocks are stock but im not sure about the rear spring???? I am in the Atlanta area and would appriciate any info!! I am a gmc technician help me with the bike i can help you with your car!!!LOL TKS, Cliff

I appriciate the help. I do have a question though. I understand the rear shock and spring but what is a fork spring? can i change springs internally of the front forks and if so there is adjustment screws on top and bottom. Does thet mean 4 springs internall? Dont mean to sound so uneducated here but i am a car mechanic!! Tks, Cliff