It's not a gap filling problem, the issue is shaping the leading edge and moving the dowel holes in the fuse. He needs to take a lot more material off the LE to get the correct shape. Enlarge the holes in the fuse so the wing can move. Look where the LE touches the fuse, then take some off that spot, put it back on the fuse, see where it touches, take more off, repeat until you have a good fit, then worry about the very small gaps and fill them.

To fix the dowel holes he'll need to enlarge the holes so he can refit the wing (above) then glue in a new piece of ply, over the enlarged hole with a hole for the dowels. If he has access to the dowel holes it make it very easy because the new piece of ply can be glued in with the wing on the fuse.