If your son does not qualify as your dependent for 2019, then you can't claim anything for education expenses paid in 2019.
He worked 6 months and is filing separately.
That tells me he was married on/before Dec 31 of 2019. One requirement for him to qualify as your dependent if he is married, is that he must file as married filing separate. Additionally, his spouse can "NOT' claim the deduction for him on her return, and he must select the option for "I can be claimed on someone else's return" when your son is completing his tax return.
Additionally, assuming he was under the age of 24 on Dec 31 of the tax year, he must also *not* have provided more than half of his own support for the *entire* year.
It's important for your son to understand that when a married couple files separate, they both "AUTOMATICALLY" disqualify themselves for a large number of deductions and credits they would have qualified for if they filed joint. This includes all education expenses, as well as any deduction for interest he may have paid on a qualified student loan.
If kids are involved, they will not be able to take the EITC or the deduction for childcare expenses.
So just be aware of all that they will potentially lose, if you want to claim him as a dependent on your tax return.
