Try boiling some water (Really bring it to a boil, not just "hot"), and dump t he head it... Judging by the plastic, from what I can see, it's probably basicaly the same pliable plastic Hasbro uses on SW figure heads and limbs. the pliable plastic's used so that figures don't become "loose" at their joints (remember how G.I. joes used to get? Or a vintage 3PO figure?).
Basically, you leave it set for 30 seconds or a minute even. You pull it out with tongs, try to dry it a little, but don't let it lose the "heat" it's retaiined to make it really flimsy/rubbery. Then just force it onto the body.
That should do the trick for you.