Yes. Page 58, under "4. Ability Score:" in the second paragraph.
EDIT: Can also show him any rules for gaining/losing Con bonus, like Barbarian Rage. Rage gives +4 Con, which means you gain +2 hp/level, clearly a retroactive increase. And when you lose that +4 Con, you also lose -2 hp/level, so con losses are also retroactive.
It is, I don't have my books to give you a page number. But it should be directly under the description of constitution at the start of the book. That's where it is in the SRD
SRD said:
If a character’s Constitution score changes enough to alter his or her Constitution modifier, the character’s hit points also increase or decrease accordingly.
Thanks. The main problem was he was interpreting things like barbarian increase as temporary and so only a temporary increase would retroactively work since he doesn't think it's logical the way I was arguing. Figure that logic out. I wish I had thought to look at the leveling section, I've been searching the DMG and trying to get the rules compendium to work but my .pdf version keeps shutting me down. Thanks again.
If he's having trouble mentally justifying the "retroactive" gain in hit points, point out that they're not truly retroactive. The Con's bonus applies to the number of hit dice you have right now regardless of what your Con score was when you gained the hit dice. The Con bonus is very much about the state the character is in now and is always calculated on the number of hit dice you have now.
Contrast that with the skill bonus for Int. In this case, it affects the number of skill points you get now only. It doesn't have a retroactive element because skill points you received for earlier levels were spent then on skill ranks, back when you weren't quite so smart.
At least that's how I conceptualize them. Makes sense to me.