The issue here doesn't seem to be his generosity as much as your focus on it.
He doesn't have an issue with his spending. You do. The only thing we're able to change is ourselves.
I’m tired of him allowing his friends to use their pity to financially take advantage
There is nothing you can do about his actions, only yours. Acceptance of the fact that things are not how you would like them and that this is beyond your control is a really good place to start finding relief from your stress about this.
If this isn't what you want in a long time relationship, it'd be a good idea to start doing some investigative self-work to discover what you really would like in a partner, let go of this one, and leave room open in your life for someone else.