son's father receives income from rentals

What is the name of your state?
New Jersey

My son is 3 yo. His father lives in Illinois (Chicago). He has only seen him once (He came to sign birth certificate) We had an informal cs agreement and until this month he has always sent the money (although late). I would like to formalize our agreement because frankly I am tired of chasing him down every month and hearing excuses afterwards. I was told at the monmouth county courthouse (I called) that I would have to file for custody first. We were never married. I believe I will get sole custody as he doesn't visit or call (although I beg him to whenever we speak). My question is.... He gets his income from rental property. I am not sure how many he currently owns (when I left him at 5 months pregnant he had 7 or 8 units with rents from 600-800$) he owed nothing on them as they were left to him by his deceased parents. How can I find out about his income????

Of course, you DO understand that the rental "income" is offset by the following management expenses, even without a mortgage:

property taxes and any other local taxes
fire and casualty insurance
utilities for common areas (and sometimes heat)
water and sewer charges, if applicable, otherwise well and septic costs
snow plowing and snow removal from walks, where applicable
lawn and tree service
legal costs for evictions, etc and bad debt from skipped tenants
credit reporting and advertising for replacement tenants
furnace and AC maintenance
maintenance expenses and reserve for long-term maintenance ("a budget" reserving for long term replacement such as roofs, furnaces, surface asphalt/gravel for parking areas, painting, carpet replacement, and appliance replacement, if applicable.
and possibly management fees, if a third party manages

The rental amount is only the "gross" income. His real property income is what remains after those standard costs are deducted out. You will need to file to establish paternity legally, if you have not already done so. And file for custody and CS. Several years of tax returns and his business profit and loss statements should be requested. You may be better off gettting your own attorney, as his income is more complicated, and he is likely not garnishable.