You can read online or download all three volumes of Nonnus' Dionysiaca on the internet archive here
. The quote in question is in volume 3, Book 41, lines 204-211, here
I found this note on wikipedia regarding Ares as the boar, though I do not know if it should be trusted nor can I find these books online:
According to Nonnus, Dionysiaca 42.1f. Servius on Virgil's Eclogues x.18; Orphic Hymn lv.10; Ptolemy Hephaestionos, i.306, all noted by Graves. Atallah (1966) fails to find any cultic or cultural connection with the boar, which he sees simply as a heroic myth-element.