I think I bugged Li :(

[Perfectblue]

So I got Li's song and before leaving the cave I put his helmet back on. Now whenever I play his song he does not appear and in addition playing the bind song fails to remove his helmet. There are also two Li npc's, one at the entrance to the cave just swimming around with a helmet on and one at the very back where you first corner him.

[Chibi]

You just got the same glitch as this guy. Did you save beforehand? Try going back to Naija's home and singing the Li song (or is it the bind song?) next to Li's helmet. That's the default way to bring him back if you've lost him. Good luck! 

[Zeke]

I think the way I fixed this in my case was to go to one of the two Lis and press the interact button (right click for most players).  Haven't looked in the other thread to see if it mentions this there.

[elZorro]

After you take his helmet off the first time, you interact with his helmet to take it off or on again, you don't bind it.
You put his helmet back on if you don't want him to follow you, he's not supposed to come when you sing his song.

[Chibi]

Answering your first sentence: that's what I thought (I put it in parentheses and as a question to emphasize my uncertainty).

Answering your second: Li comes back when you sing his song next to his helmet in Naija's home if you put his helmet on in the Veil and left him there (or vice versa mapwise).

[silverflagon]

The only time I leave Li behind is when I have tried to beat the Simple Simon boss and I have done exactly as Chibi says.