What is happening to Lord Buddha after "parinirvana"? We all know that after Lord Buddha's parinirvana, he will never be born again. So my question is, if Lord Buddha does not have a rebirth after parinirvana, what is actually happening to him (after death)?
Nothing

Because there is no person to happen anything, and there is no samsara. So that's The End. When a person achieves nirvana , there is no person anymore. Death is not the end of a person, nirvana is. After achieving nirvana, 'he' can help others to achieve nirvana , and just wait for the physical death.