...and all that happend was he lost the dawnguard items and quests, and he finished it to the end. (I don't understand what you mean here !!!)
> when it was already put on my save < - That's the key here...
Once you delete Dawnguard, any items (crossbow, characters, etc) will no longer be in the game.
Nothing to do with Dawnguard will be available in the game once it's deleted.
If you try to load a save which was made while Dawnguard was active, but after you have deleted it,
you will get either an error that the save relies on content no longer present, or else the game will crash.
To properly uninstall (delete) Dawnguard follow those steps:
1. Go with your character somewhere in the Skyrim worldspace. Maybe just outside Whiterun or a similar location.
If you have any Dawnguard items in the player character inventory, sell them (because although you can keep the money you sell them for)
(you can't (shouldn't) keep the actual item once you delete Dawnguard.)
2. Save the game.
3. Delete Dawnguard.
4. Now restart the game and reload the save you just made.
5. Now save the game again (reload and play...)
GOOD LUCK !!!! (and may the force be with you...always)