Well from what I am finding the most you could do is demand for more inspections.
Curiously in my researches on this I found that Assad might technically not be considered a war criminal under international law at all even if we operate under the assumption that indeed he did use chemical weapons (which keep in mind he denies.) Pretty much the crux of the matter is that technically it's not a war crime unless they are used in a war of "international character" or unless Syria were to become party to some recent amendments to the statute linked below, which it is not a party to and neither is the US nor Russia.
https://en.wikipedia.org/wiki/Amendments_to_the_Rome_Statute_of_the_International_Criminal_Court
Curiously, if we pretend that Syria, the US, and Russia did adopt these amendments, then technically the US would be guilty of a war crime, a "crime of aggression" for the missile strike against Assad and also for helping the rebels to undermine a sovereign nation.
So I guess in summary, Assad can technically drop as much chemical weapons he wants to so long as they are inside Syria and it is not a war crime and Trump can send as many salvos of missiles into Syria and give as many weapons and money to jihadists as he likes and it is not a war crime either. I guess it pretty much sums up pretty well the great oxymoron of the notions of international law and laws of war.