I think he should get a save...
I also think it's pretty low down to try and kill a PC using a 'dirty trick' like this. I can't see many players having this happen to them NOT being exrtemely pissed and basically thinking 'Why is the DM picking on me'. Evne if it is within RAW, which I don't think it is, it's a simple case of you wanting to kill a player/s, not you wanting to run a fun, heroic adventure for/with you PCs.
That said, I agree with hypersmurf's interpretation that he would be allowed a save since the spell isn't [harmless].
The text in the book doesen't say 'each time someone casts a spell on you, you must CHOOSE to forego your save or try and resist it.' Choosing not to actively resist a [harmless] spell is NOT the same thing as willingly failing a saving throw against it.
Pretend its like makeing out....Boy starts to makeout with a girl, they kiss and touch, a little bit of harmless fun, doesen't mean she's agreed to go 'all the way'. If boy's hands start to wander too far south, then she's gonna respond with a saving throw to his right cheek! Same thing with magic...guy casts a spell on you, it 'feels' harmless so you're not actively resisting, but your also not actively making yourself more vulnerable to it...then the magic starts trying to do sick and depraved things to your soul and your body decides it doesen't like that so much, so you resist. If mechanically you had to decide to make saves or not BEFORE spells were cast, then I would agree that fighter asking for a cure spell = autofail....But there has got to be some sort of tangible interaction between the time a spell is cast on a person, the time the spell takes to 'sink in' and start TRYING to work, and the time they decide to make a save or not.
If you want to use a tactic like this, then you should alway's ask your party for EVERY spell they cast on each other, every time, and thats lame. The PLAYER might not know that he has been forgoing a WILL save a thousand times, but the CHARACTER should notice when the WILL save he has been giving up so many times suddenly feels like a FORT save instead.
Also for what its worth, Enlarge person is a bad example of a spell thats good to sneakily trade for a save-nah- die spell....It's a 1 round casting time. Even without spellcraft its gonna be pretty obvious that the casting time is way off.