When you start to enter a dependent, the first screen asks "Who do you support?" If you select "Another person" it then gives you a drop-down list to select the relationship. The last choice in that list is "Other." See the screen shot below. You can select Other and then proceed through the questions to determine whether you can claim him as a dependent.
However, one of the requirements to claim someone as a dependent who is not related to you is that he lived with you for the entire year. You didn't say whether the two of you live together. If your boyfriend only lived with you "for months of last year," or did not live with you at all, then you cannot claim him as a dependent.
![Dependent other.png Dependent other.png](/community/image/serverpage/image-id/40660iD8D78A692D471CF6/image-size/large?v=v2&px=999)