I had acne ages 10-38.... Still have greasy skin.
Biore watery essence is one of the few truly non-greasy, yet non-drying SPFs out there.
It's necessary to avoid drying out oily, acne-prone skin, as that tends to encourage the skin to pump out more oil to compensate, leading to a vicious circle.
Make sure his cleanser isn't drying. Counter-intuitive as it might seem, after years of using gel cleansers I discovered the best solution to be oil cleansing. The larger oil particles of an oil cleanser (or even just plain grapeseed oil) act as magnets for the smaller sebum oil particles, and take them off the face without any hard cleansers, so the skin isn't left dry. All you do is massage the oil in and then use hot water and a muslin cloth to take it off.
For a treatment, I tried the lot bar Roaccutane, and found none of them very effective long-term, except for niacinamide, which worked best for me and cleared most of the spots.
Stress can really exacerbate acne, so helping him learn ways to keep stress lower could really be worthwhile. Also, some people find low carb diets help too. I'm not sure if it made much of a difference for me, but it might be worth trying.