I have the opposite problem where Halsin will not follow in elf form. Restarting the game, reloading from a hard save, having Halsin leave and rejoin the party all do not work...
I just had this, too, at the beginning of Act 3, and tried the same things as you. Only using wildshape made him start following again. And he continued to do so even after he then turned back into an elf, though probably not for long on recent form.
The good news is that I have had acknowledgement of my bug report from Larian, and they asked me a couple of follow-up questions, so that suggests it's being looked into.
EDIT: Just played around a bit more, and interestingly turning into his personal Cave Bear form didn't make him start following, whereas turning into an Owlbear or Polar Bear did. (I somehow can't feel its right to make him turn into something that's not even slightly a bear
).
EDIT AGAIN: No, ignore me. The Cave Bear does work to make him start following again, it's just that he happened to be standing in the back room at Arfur's in Rivington when I was playing around with this, and it seems as though the Cave Bear form won't fit through the door!