Can I claim my boyfriend as a dependent? He lives in my house with me, had $0 income, and is a citizen. Only I had an income. but all expenses were paid from a JOINT bank account that I funded.

To claim a non-relative as a dependent the must meet all the requirements under the Qualifying Relative rules.

To be a Qualifying Relative -

1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.
2. The person either (a) must be related to you or (b) must live with you all year as a member of your household.
3. The person's gross income for the year must be less than $4,300 (social security does not count) in 2021
4. You must provide more than half of the person's total support for the year.
5. The person must be a U.S. citizen or a U.S., Canada, or Mexico resident for some part of the year.
6. The person must not file a joint return with their spouse.

however, his parents even though they don't support him may have  higher priority than you in claiming him'

to qualify for them to claim him  he would have had to live with them for more than 1/2 the year

and either be under 19 or under 24 if a full-time student