Wait till he discovers Lisp. Tcl's flexibility combined with actual types (and not stringly typed). That's how I got seriously into Lisp: reading about the "Tcl Wars" and then trying out Guile.
That said, Tcl/Tk is still the fastest way I've seen to go from zero to GUI. Not even Visual Basic beats it in terms of quickness and simplicity.
Rebol is the only thing that really blows Tk out of the water from a GUI perspective. It's really a masterpiece of engineering. It's all based off of blocks and is really nice to use.
And cl-icebox (https://github.com/VitoVan/cl-icebox) is a build system, that can compile your Ltk apps into standalone executables for easy distribution.
That said, Tcl/Tk is still the fastest way I've seen to go from zero to GUI. Not even Visual Basic beats it in terms of quickness and simplicity.