He wont ask me to be his girlfriend

me and my SO have been together for about four months now and he still hasn’t made it official. i stay at his house four or five nights a week and we have been on some dates and have even talked about living together next year when his lease is up. i’ve brought it up to him a few times and he always says he doesn’t care for or really understand labels. he knows that it bothers me, i want something to call him and i want to be able to post it etc. he’s never denied being with me, we both have always said yes if anyone asked. i just don’t get why he doesn’t want to label this? (and no, he isn’t seeing anyone else and neither am i)