Sagging rear suspension 1990 cr80

[aus stretch]

Hi to all from a newbie.

My son has just purchased a 1990 cr80.
The only prob he has at the moment is the rear suspension rides very low.
I might add that I know nothing about these things.
Is there a quick adjustment that can be performed by 2 novices or is this a job for someone who knows their business?
We have topped up the air in front and rear suspension, front soilified nicely but the rear still sits low.
Any help or links to visual guides appreciated

 

[CRazy250]

the rear shock probably needs to be rebuilt and recharged with nitrogen, and it needs to be set up for his weight.