You can if the tests below are met. Since he had no income, the partial disability doesn't matter. Select "another person" when asked about the dependent, and you will see "grandchild" on the list (see screenshot)
1) You provided more than half
of their financial support. More info
2) They made less than $4,050 in gross income
during 2016 unless they are a
qualifying child (under 19 OR a full time student under 24
OR permanently disabled)
3) They live with you or they are related to
you.
4) They are a U.S. citizen, resident alien,
national, or a Canadian or Mexican resident.
5) They aren't (or won't be) claimed as a
dependent by someone else.
6) They aren’t filing a joint return with
their spouse.
7) You are not being claimed as a dependent on
someone else's return.
