I don't want my bf to wear briefs
What makes you want to tell your boyfriend what underwear to wear?
Because briefs are particularly unsuitable for a man with a big butt
Your boyfriend can choose whatever underwear he wants to wear. Maybe you can focus on what is flattering about them instead of the negative aspects.
I do pay attention to other things like big shirt or smooth hips.
That's great. You shouldn't tell your boyfriend what to wear, but you shoudl compliment him on what you like about him.