Bill raised an eyebrow because it couldn't only be that his son had a low tolerance? Could it?
"You're mum wasn't either," he said, patting his son on the shoulder. "Oh she'd be around, cradling her glass with the rest of the family but she never really drank much, not like the rest of us."
Giving his shoulder a fatherly squeeze he let go. "Don't worry about it. You're just eighteen. You'll get the hang of drinking eventually. You're a Weasley, after all. Or if you're a Delacour in that respect, you'll learn to pack a sobering potion - or go your mum's route and limit your alcohol intake."
He watched his son. "Sure that's it?" he asked. "Because I'm sure Jamie can get you a sobering potion if you want one."