Deductions & credits

Your son who was in high school in 2018 can be claimed as your dependent for 2018.  The fact that he lived away at school was just a "temporary absence"--so you can say he lived with you for the whole year.

If you did not claim your son on your 2018 return you can amend that return to claim him

What does it mean to "amend" a return

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed?...

And--you did not ask, but if your son was a full-time college student in 2019 you can still claim him as your dependent for your 2019 return.

WHO CAN I CLAIM AS A DEPENDENT?

You can claim a child, relative, friend, fiance (etc.) as a dependent on your 2019 taxes as long as they meet the following requirements:

Qualifying child

• They are related to you.

• They cannot be claimed as a dependent by someone else.

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

• They are not filing a joint return with their spouse.

• They are under the age of 19 (or 24 for full-time students).

• No age limit for permanently and totally disabled children

  •      They live with you for more than half the year (exceptions apply).

Qualifying relative

• They don't have to be related to you (despite the name).

• They cannot be claimed as a dependent by someone else.

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

• They are not filing a joint return with their spouse.

• They lived with you the entire year.

• They made less than $4200  (not counting Social Security)

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

When you add someone as a dependent, we'll ask a series of questions to make sure you can claim them.

Related Information:

• Does a dependent have to live with me?

• What does "financially support another person" mean?

• Can I claim a newborn baby?