"what he's thinking about when he reaches orgasm, coz it'll set his sexuality."

NO NO NO NO NO NO NO

This is not true at all. Stop telling him this lie. Even if you believe it it is still a lie. This would mean if I started purposely thinking about something else my sexuality would change.
I was a boylover before I ever masturbated and so was everyone other BL I have ever talked to about it. You don't masturbate thinking about a boy and magically become a BL. Sexuality does not work that way.