My 7yr old is still not even 20kg...try and relax about weight unless you have a suspicion of some underlying health issue.
Genetics plays a big part in kids weight, if you and your DH were light when you were young, he probably is the same. My DS1 in particular is his father's son, eats like a horse, adult portions and still doesn't put on weight

A 4yr old is pretty active. My DS2 is 4.5yrs and is hovering close to 18kg only due to the fact he is a big pasta eater and meat only.

As Trill has mentioned, give him full fat products if you are truly worried.