At first I thought it might be a sensory issue, but you said the reason why this boy is like this is because he got scratched from a button. So I am guessing there is a fear attached to the issue at hand.
If it was my child, I would not seek a therapist based on what you told me. Rather, I would drop the subject by avoiding shirts with buttons for a long time.... like a year and NOT talk about how he hates to wear anything with buttons and NOT talk about how he got scratched from a button in front of the child.
My guess is that is memory about being scratched from a button will fade if the topic is not discussed. Then after a year or so, the boy and his mom could look for shirts and she could say... "Help me pick out some shirts for you... they all look so nice.... which two do you want me to buy for you today?" (And have the choices be from buttoned
shirts) Hopefully he won't mention that they all have buttons on the shirts... I think it's worth a try.
Side story that is similar: My son decided he was not going to wear shirts with no sleeves (tank tops) anymore. He would not explain why, but it appeared that the reason was because he felt he was exposing too much of his body... like it was a private part or something (his armpits)It seemed like he feared showing too much skin.
Well... the more I focused on it by saying.... "oh it's going to be 90 degrees today, why don't you wear a tank top?" ... the worse it made him feel. I decided to not buy anymore tank tops and didn't talk about the issue anymore.
I thought... okay no big deal... he prefers not to wear them. WELL... just last night he asked if he could wear his baseball tank top to bed. He also wanted to know wear the rest of his tank tops were. I said I gave them to his younger cousins because they were getting too small for him. So he asked if we could go to the store and buy some new ones:)
So.. with my personal story, I felt compelled to share and tell you that with time, I think your friend's son may outgrow this issue of no buttons. The only thing is if he has to wear buttons for say... a private school (uniforms). So I hope he does not HAVE to for any reason.
My son's "little hang up" (I don't say that to him) about the whole tank top issue has lasted well over a year. And for the last three years he has insisted on wearing a swim shirt when he wears his swimming trunks. (even if it's just in his backyard) He thinks by showing his upper body, he looks naked. He does not want to expose his upper body to anybody and that's okay even if most of the other boys aren't wearing swim shirts... hey at least he is being protected from the sun's harmful rays, too.