His tree is probably an American Walnut but if it's a Black Walnut what might
a
Tree like that be worth these days?


Nothing. Other than its asthetic value or value that can be placed on it for
insurance purposes, black walnuts in the landscape arent used for thier wood.
The main reason is that outside of thier natural environment, trees are subject
to homeowners who like to put nails and all sorts of wierd stuff into thier
trees, and prune them improperly(increasing wound sites that lead to decay).
They also do not acheive the necessary size in the landscape to produce the
veneer quality wood that is used for furniture and such.

Toad