"He walked past it" and "he walked past him" mean that someone male traveled on foot near to and then beyond something: EXAMPLES: Tony walked past the library. Tony walked past the man who was standing on the corner. "He walks it past" and "he walked him past" are different constructions with a different meanings. EXAMPLES: Tony walks the dog (it) past the library every day. Tony walked his father (him) past the house where they used to live.

A veces puede ser separable. Por ejemplo, podemos decir, “He walked past the house he was looking for” o “He walked two blocks past the house he was looking for."

Absolutely unseparable.