Well if he won't resettle you're in the same position you were before, right? Nothing lost. I don't know if WIWO will work, or if it's right for Liam - just telling you that my daughter WOULD go back to sleep after a short nap, but when we were in the transition it would take her a long time to resettle.
I'd bet on OT rather than UT since he's doing short nights and short naps, no? What do you think?