Joint ownership 4 plex with adult child (18). TAX how to?

[Tristan Cortez]

2019 I purchased a 4 plex with 18 yr old son to get started real estate investing. Son is primary on the loan (so his ssn listed on 1098 but it is a joint ownership mortgage. he is full time student so not making much (any) income. we have not established a partnership or an LLC. Can I just add this to my taxes as a new rental property without a partnership or LLC or do I have to establish one by completing the irs form? How do i do this with his ssn on the 1098 Mort. form but I am paying for everything?