Is it possible that my 5 yo's eardrum ruptured in his sleep?

Probably yes: If your child has had a middle ear infection, during sleep the venous pressure in the middle ear is raised and it is possible for a rupture of the ear drum to occur with resultant drainage. An appropriate diagnosis by a pediatric otologist is necessary for the corrected diagnosis and treatment.

It happens: Some germs that trigger middle ear infections will come on quickly & create enough pressure pushing out on the ear drum that it can rupture. Pneumococcus is a common germ known for that action. The pressure may create pain which resolves some after the rupture. Drainage in the canal is often observed. This differs from the mechanics of a drum rupture from loud sounds (out to in pressure)