Cant get anywhere with Connacht

i'm fairly new to CK2 and am picking up the basics. anyways i started my game as Connacht and got a lucky strong claim on my neighbor who i immediately went to war with, crushed his army occupied 1/2 of his province and repetitly destroyed his forces over and over again. but because my Max troops was only 500 and his castle has 590 i cannot seige it thus province is only 1/2 occupied even though he has no troops and i could let him respawn and kill them i cannot get higher than 96%. so i have no idea what todo. sign a white peace for a war i clearly won..... frustrating