[metalalchemist]

The super glue trick should do just fine if you do it carefully. I have done it many times with a lot of loose figures. Just turn him upside down, shoot a couple of drops, move the head until dry and repeat until tight.
But if you do wanna take the head off, boil and pop is the method: Boil a bit of water in a recipient in the microwave and dip the head in it for a minute. This will soften the plastic enough for you to pull the head out (just cover in water the head and joint parts so that nothing else becomes soft and loose when pulling it out)