"him not accepting the proposal is really surprising"
You didn't actually state your question, but I assume it's about the acceptability of this example, in particular the use of him. The simple answer is yes - grammatically it's acceptable, though the informal accusative him is less likely than the more f
If Him not accepting the proposal is correct, then Him doesn't know it is, too. The object form him is perfectly grammatical when there is something before it that requires an object form. The most common such cases are prepositions and many verbs. These would be perfectly all right:
If Him not accepting the proposal is correct, then Him doesn't know it is, too
Not so. *Him doesn't know it is an entirely different construction. It's a finite clause so the nominative he as subject is obligatory: He doesn't know it.