In certain dialects of AmE, notablenotably urban black dialects, "wanna" is correct (in that dialect) 3rd person singular. From the article on African-American Vernacular English on Wikipedia:
Present-tense verbs are uninflected for number/person: there is no -s ending in the present-tense third-person singular. Example: She write poetry ("She writes poetry").
Standard:
He wants to try the new ice cream.
Urban (African-American):
He want to try the new ice cream
So it would not be surprising in the contraction you mention that the urban dialect would yield
He wanna try the new ice cream.