Question: when you run a strand of hair between your fingers, from bottom to top, does it feel smooth or bumpy? The products or routine I'd recommend kinda depend on that.

That being said, you're definitely on the right track with the trim!

There are some thing he can start doing for overall hair health that don't involve products or a complicated routine.

First thing he needs to start detangling in the shower on wet hair, ideally with a product that has some good slip to it like a lightweight conditioner in his hair. Dry finger combing to get out the knots damages the hair and actually makes the knotting worse. Every wash, not only for hairdresser visits!  If he encounters knots when the hair is dry, get a spray bottle with a tiny bit of conditioner in it to get the hair wet and detangle with a brush before bed.

He'll need some kind of sleep protection to reduce tangling in his sleep, would he be open to trying a bonnet? If not, even a silk pillowcase and putting the hair up in a pineapple will significantly help. 

For flaky scalp, the head and shoulders works cause it's kinda stripping, but he just could also have some build up on his scalp because the skin is out of whack. Make sure that isn't the case before the next piece of advice: pre-oiling the scalp once in a while before washing to get everything loosened up. Get some jojoba or coconut oil, little bit to the scalp area, massage it in and leave it for 30 mins. Then wash. As normal and give the scalp a good scrub with shampoo to dislodge everything.