Well if he doesn't like his naturally curly hair then he should flat iron it. He's the one who has to wear it. If he's more confident with straight hair then there's not a lot you can do to convince him to stay curly.
Try reviewing some pictures of curly guys and see if he finds one with similar hair color and shape that he would want to achieve. That's a good starting point...going after an "inspiration piece" hair style.
Either way the routine for healthy long term hair is to cut down on shampoo everyday and increase conditioning. He can try shampoo maybe once a week and use conditioner only during his daily shower. That's a simple starting place and you can learn more about what types of conditioner or shampoo works best for him over time.
Personally I say NO to conditioners and shampoos that contain ingredients than end in or contain dimethicone, dimethiconol, sulfate ingredients, sodium hydroxide, or -hydes.
IF HE KEEPS HIS CURLS:
If he tries to improve his curls then he will need to use a hydrating/moisturizing leave-in conditioner after every wash.
IF HE DITCHES CURLS THEN:
If he decides to flat iron then he will need a ceramic flatiron and a good dimethicone containing heat spray. He should wash his hair and use a conditioner that contains dimethicone. Then add more conditioner combing all the way through and then let dry completely. After it dries then he can start spraying with the heat guard and be sure to always spray each section of hair right before it goes through the straightener. Use a comb to guide and spread the hair as it glides into the flat iron. Don't flat iron more than once and don't flat iron too fast. Also don't flat iron too slow or he might boil the water in his hair which will damage it beyond repair.
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