If you did not receive your first and/or second stimulus payment or if the amount received is not correct, you can claim it on your 2020 tax return as the Recovery Rebate Credit.

In TurboTax Online, to claim the Recovery Rebate credit please do the following:

1. Sign into your account and continue from where you left off
2. Click on Federal in the left-hand column, then on Federal Review on the top of the screen
3. On the next page titled Let's make sure you got the right stimulus amount, click on Continue
4. Follow the interview
5. TurboTax will determine whether you are entitled to any additional stimulus
6. Any stimulus amount remaining due to you will show as a credit on line 30 of your form 1040.

I did this but it is saying I received what I should have. 

So can I list him as my nephew even if he is my fiancee's nephew? It allows me to claim the stimulus then? 

the child must be your dependent on your return and under age 17 in 2020.

then you enter what you got, which would be short $1,100.

Line 30 will show the difference.

See IRS Pub 501 for who can be a dependent on your tax return

ok so per this from that document, he would be a qualifying child right?  :

 Child of person not required to file a return.
A child isn't the qualifying child of any other taxpayer and so may qualify as your qualifying relative if the child's parent (or other person for
whom the child is defined as a qualifying child)
isn't required to file an income tax return and either:
• Doesn't file an income tax return, or
• Files a return only to get a refund of income tax withheld or estimated tax paid. 

So how do I get turbo tax's program to acknowledge this without me putting him as my own nephew in it. I mean, he is listed under his uncle (my fiancee) so would it then be that he is his nephew?