Hi Rhys,
It looks like your son grab (and was grabbed in turn by) a species of Rubus, probably Rubus cissoides. There are many native species of these hooked lianes from the same family as rasberry, commonly referred to as bush lawyer. The fruit are edible, but flowering usually occurs on the edges or canopy of other trees where there is more light, and birds often get the ripe fruit before people.
I'm afraid the fact sheet on our website has not been completed yet, but you can see more images here: http://www.nzpcn.org.nz/flora_details.aspx?ID=1263