I figured out how to play His Theme on the guitar and thought I might as well share it here since there aren't enough undertale songs written out in this format.

If you're completely new to the webfishing guitar, this tutorial for Home by VenomOrg is very helpful for learning how the guitar works. I recommend at least reading their explanation of how the guitar works before continuing with this tutorial.

This song only needs 2 chords saved, so if you have space in another song's slot it's probably a good idea to just tuck it in there instead of using up a whole slot. Just remember that your '1 & 2' in the instructions will be whatever two numbers you put the chords in.

1 Q T R W - E E - R
W R W - E E - R
W T R W - E E - R
2 W E T R E R

It's pretty short but most of the song is just this looped.
I might add the ending bit later if I figure out how to play it.

If people are interested in Hopes and Dreams or Fallen Down I've been working on those songs as well ^^