No one word denotes the frequency you seek in another word for 'volunteer', although some words suggest or connote it in the context you give with your example.
Among those words are
I'll rule out 'philanthropist' for your answer because overuse of 'philanthropist' in mass media has cheapened the word by associating it too closely with a 'giver of money' in stead of the truer 'supplier of resources'.
A "reliable altruist" is one who practices altruism frequently:
1. Disinterested or selfless concern for the well-being of others, esp. as a principle of action.
[Emphasis mine. "altruism, n.". OED Online. September 2015. Oxford University Press. http://www.oed.com/view/Entry/5857 (accessed November 19, 2015).]
So also a "reliable humanitarian" is
- A person concerned with human welfare as a primary or pre-eminent good; esp. a person who seeks to promote human welfare and advocates action on this basis rather than for pragmatic or strategic reasons; a philanthropist.
[Emphasis mine. "humanitarian, n. and adj.". OED Online. September 2015. Oxford University Press. http://www.oed.com/view/Entry/89276?redirectedFrom=humanitarian (accessed November 19, 2015).]