Check your local ordinances first. The easiest way to shut him down would be "It's illegal".
IF it's legal, show him what size enclosure it would need. Make CERTAIN you make it clear that if he gets it, HE has to handle it and clean it and feed it because you WILL NOT. Not waffling about how you'll "have to". He has to. Tell him if he can't handle and clean your BP for a month, he obviously has no business getting a anaconda now.
He might eventually make a good giant keeper, but right now, he sounds a lot like he's letting you be his enabler and as long as you're there and willing to take care of the pets, why should he bother? He can buy them or keep them and you'll do the work because you want to see the pets get proper care.
If he says he's getting one anyway, I'd find somewhere else to live and MOVE. Sooner or later, he's going to cause a serious issue if he ends up with a giant and no ability to handle it. It will only take one instnace of not latching a cage door and then you'll have serious issues because neighborhoods do not easily forgive loose giant snakes. Poor press all around.