KTM mini suspension 45lb rider

We have recently acquired a 2015 KTM SX50 MINI. I noticed the rear seemed really soft or the forks stiff? The bike seems unbalanced, like the rear squats too much, or when I jounce it from the peg the rear dips more then the front. I was curious if any reccomended mini suspension service I should do to get this right for my little guy. The spring reads 75 120 BTW not sure if that's the right size for us? Any advice on mini bike suspension setup/service/revalve I'm all ears.