Hi M.,
I have not read the responses you've rec'd thus far, but i wanted to share my opinion, so forgive me if there is repetitive info.
I would offer a bottle, and if he doesn't finish it in 20 minutes, he doesn't want it. I actually warm his milk in the micro and he sits in his sling to drink it... if he doesn't finish it, i dump it.
Here's my thoughts on this.
Yes, milk can get costly at darn near 3-4 bucks a gallon when not on sale but (it's still cheaper than gas!!!HA!), you paid a lot more per ounce when you were giving him formula. AND, the first time your boy throws up from drinking spoiled milk, you will hate yourself for wanting to save the money, and at that moment, seeing him vomit and feeling yucky, you will realize that it's just not worth the cost savings.
Now i'm not suggesting you are willing to risk him being sick... which is why you asked right? but when you are tempted to put it back in the fridge, think twice.... Personally... i cannot stand milk that is not ICE COLD :) After about 10 minutes sitting out (even my glass i have with dinner) i cannot drink it :)