There is only one spell that actually works, and it has nothing to do with the size of any guy's wrench I can assure you.
Trust me, if there was a spell to make a guy's dingle-dangle bigger you wouldn't have to ask about it. Everyone would know it, and every guy would be walking around with their friendly weapon slung over their shoulder. I'm pretty under-endowed myself (he's small but he knows kung fu), and I'd have a sausage the size of my right arm if such a spell existed.
Maybe your boyfriend doesn't have a small organ, maybe the problem is that he's performing in a cathedral. Try some kegal exercises or something.
Oh wait, are you wondering what that one spell that works is? Because it's not a secret, so I can tell you. You've probably already had this spell cast on you several times, and it really does work every time. Still wanna know? You sure? Positive? OK OK, here it is...
"When you grow up and have kids of your own, I hope they act just like you."
There it is, that's the spell that works. Ask anyone who has kids if you don't believe me.