Companion would appear like he was gay, as would life partner. The fact that you were together for a year would not dictate a life partner regardless. The most tasteful addition would "Friend" while it does not specifically dictate a relationship it does imply there was someone that was very close. At the base of all relationships before marriage you are always friends so it would be appropriate and tasteful.
You can also consider Significant other, though it could be interpreted as a lifestyle issue. Of course you could put Beloved Brother, Uncle and Boyfriend but it is a little tacky on the headstone.
At the end of the day, who were you to him............were you his friend, his partner or his companion. While the word friend may seem less loving the reality is you were his closest friend in ways no others were so using the term friend fits what you need and what you were to him.