Self employed? How do I file

You would be considered self employed so you report income on Sch C. 

Yes, you are self-employed.

To enter your self-employment income click the following:

1. Federal
2. Income and Expenses
3. Show More next to Self-Employment
4. Start next to Self-Employment
5. Yes, to you have any self-employment income or expenses
6. Enter the type of self-employment work you do and you can select that you use your name and address for your business
7. Click through a few more screens and you will get to a screen that says "Your XX info"
8. Click Looks good
9. Then you will come to a screen that says Let's enter the income for your business.  It will give you the option of 1099-NEC or Other self-employed income including the 1099K , cash and checks.  Here is where you will enter the money he gave you
10. If you do not select one, it will not let you enter your income
11. Enter your self-employment income on the next screen
12. Continue and on the next screen click add expenses

In the expense section, if you drove to different job sites, you may be able to take mileage or vehicle expense deduction. 

This income will be reported on Schedule C which is part of your overall tax return. This also means you will be subject to SE Tax.  If you did not pay SE Taxes in 2023, and if you took over his business, it may be a good idea to make  quarterly estimated payments to cover your SE taxes of 15.3% and avoid penalties.