Answer: Adverb clause.
Explanation: an adverb is a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree, etc. An adverb clause is a group of words that function as an adverb. In the given sentence, the phrase "whenever he has time" is an adverb clause that is modifying the verb "volunteers" by indicating time.