My adult step son didnt make any money in 2020 can I use him as a dependent? I have already filed my 2020 taxes so do I would need to file a amended return?

You can claim an adult stepson as a dependent on your 2020 return as long as they meet all of the following requirements : 

• You provided more than half of their financial support. More info 

• They made less than $4,300 in gross income during 2020

• They live with you more than half the year

• They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident. 

• They aren't (or won't be) claimed as a dependent by someone else. 

• They aren’t filing a joint return with their spouse. 

• You are not being claimed as a dependent on someone else's return. 

To claim him, you will need to amend your return.  Here is how you can  amend your return.