If AGI rejected on son's return even though same as on return and transcript. How can we file electronically and not have same issue next year?

[fanfare]

you can e-File if you kept available the 5-digit PIN you invented last year.
It is in your tax file if you imported 2017 file.

clear the  AGI, and enter the PIN in the place provided.

Otherwise you have to file a paper return via US Mail.

[xmasbaby0]

Did he file late or amend?

Your 2017 tax return shows your 2017 Adjusted Gross Income  (AGI)  If you filed a joint return then the AGI is the same for each of you.

Form 1040EZ    line 4

Form 1040A       line 21

Form 1040        line 37

If you are using the same TT account that you used last year, you should also be able to find this number in your Tax Timeline for 2017

If entering the amount from your 2017 tax return does not work, then try entering “0” instead.  If that does not work and you still cannot e-file, then print, sign, and mail your tax return.

You can get a free transcript of your past return from the IRS:  

https://www.irs.gov/individuals/get-transcript

Note: If you filed late or amended your return, your correct 2017 AGI may not be in the system. 

https://ttlc.intuit.com/questions/1944348-how-do-i-print-and-mail-my-return-in-turbotax-online

Note:  If you mail your federal return, you will also need to mail your state return.  You will not be able to e-file your state return.

https://ttlc.intuit.com/questions/3415597-what-if-i-entered-the-correct-agi-and-i-m-still-getting-an...

https://ttlc.intuit.com/questions/1944348-how-do-i-print-and-mail-my-return-in-turbotax-online