My son is filing his 2018 taxes for the first time but also late. What does he need to do from here?

[xmasbaby0]

He does not need to do anything special.  If he is getting a tax refund, there is no penalty for filing after the filing deadline.  If he owes tax due, then he should prepare his return, file and pay the tax due ASAP.  If he is filing late with tax due, he can also expect a bill from the IRS for the penalty/interest owed which will be calculated by the IRS based on how much was owed and when the IRS received the return and his payment.

E-filing is still possible for a 2018 return until October 15, 2019; after that a 2018 return must be printed, signed and mailed.

You say he has never filed a return before, so his 2018 AGI will be "zero" in order to e-file for 2018.