My Great Empire : the parellel world

it is story about a college student. who suddenly realises that he as a unique superpower to travel between the two worlds .what almost gave him a cardiac arrest was his identity in the parallel world . the parallel universe is exactly 5 century behind his original universe and there he is a crown prince,not of some great empire but a lowly city state bordering continuesly expanding empire. Any person with two brain cells knows the fate of such a kingdom "if we even call it a kingdom" but now the crow prince as got the knowledge of the future and did i mansion that he will eventually able to bring some cool stuff from future to past but the question is how would he acquire the guns, gunpowder and other weapons,its not like every country is U.S.A. .
now we have to what changes will the protegonist bring will his kingdom be destroyed or would it evolve into a great empire. How the world map would change .