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?

You don't need an LLC. Just talk to a local accountant. This sort of thing is done all the time. If you are 50/50 partners then 50% of the profit/loss will flow to your personal taxes and 50% will flow to your sons.

Thank you Jacob.  I appreciate it. 

@Tristan Cortez

You can purchase a property with another individual through an entity(partnership, LP, LLC, etc) or as tenants in common.

You should consult with an attorney on the pros/cons of each.

Regarding how to report it on your taxes, it really depends on who owns the property and is required to make the payments.
Who has title to the property?