Humans usually don't hang out high over head and in trees, so I can understand his hesitation.
I'd do a bunch of victim in tree scenarios for the next little while and start making it super fun and positive for him, so use whatever motivates him for the victim to engage from the tree. Lots of encouragement and praise on your end, as per usual, but try and build his excitement when he sees someone in a tree.
SG S-Hunter vom Geistwasser CA CGN TT (Airport Wildlife Control K9)
Zenna vom Geistwasser
"May my enemies live long so they can see me progress."