You do NOT include your FIL's income on your own tax return. If you are paying for over half his support and he had less than $4300 of income not including Social Security then you can claim him as your dependent. You will get the $500 credit for other dependents and the $1400 3rd stimulus check for claiming him. He will not receive a stimulus check since dependents cannot receive stimulus checks.
IRS interview to help determine who can be claimed:
https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent
https://ttlc.intuit.com/questions/3113432-who-can-i-claim-as-my-dependent
You say he "earned" some income----he can file his own tax return if any tax was withheld from his paycheck so that he can get a refund. If you claim him then he has to say on his own return that he can be claimed as someone else's dependent.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
