Generally, the person whose name is on the account, and it appears this may be you because you received the 1099-DIV, is the one responsible for paying the tax. Thus, if you are the owner of the account, you could pay the tax, if any, in full. However, you have stated that the account in question was jointly funded. Therefore, if you choose to allocate the dividends, interest, etc., in a way that represents your true ownership interest (you mentioned it was 50-50), then you need to notify the IRS of same. Otherwise, if you don't notify the IRS, the IRS won't know why you did not report all of the dividends.
To split the dividend income, you’ll need to fill out a Form 1099-DIV. You will list your information as the payer and the joint owner’s (your boyfriend) information as the recipient. You’ll list the joint owner’s dividend income in box 1a, total dividends or box 1b qualified dividends. Your 1099-DIV will tell you if the dividends are qualified or not.
Make sure you only list the portion the joint owner is responsible for, not the full amount. This will let the IRS know you aren’t reporting the reported portion of the dividends on your tax return. It also alerts the IRS that the joint owner and recipient of the 1099-DIV will be reporting their share of the income on their tax return. The form will need to be sent to the person receiving the interest (the joint owner) and to the IRS. The IRS also requires you to file Form 1096, which is a summary of all 1099 forms you send to the IRS.
Use Google, and the search term 2021 IRS 1099-DIV, to access a 1099-DIV.
@apunke