You can't write off expenses per se. You can claim him as a dependent if you provide more than half his total support. If he is your dependent you can claim any medical bills you pay for him.
You can claim a child,
relative, friend, fiancé (etc.) as a dependent on your 2018 taxes as long as
they meet all of the following requirements :
- You
provided more than half of their financial support. More info
- They made
less than $4,150 in gross income during 2018 unless they are a qualifying
child.
- They
live with you or they are related to you. (Your relative must live at your
residence all year or be on the list of “relatives
who do not live with you” in Publication
501.)
- They are a
U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
- They
aren't (or won't be) claimed as a dependent by someone else.
- They
aren’t filing a joint return with their spouse.
- You are
not being claimed as a dependent on someone else's return.
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.
From <https://www.irs.gov/publications/p501#en_US_2017_publink1000220942>
