If he qualifies as your dependent, sure you can. He will qualify as your dependent under the "qualifiying relative dependent" rules. Basically, so long as you provided more than half of his support for the entire year and he has less than $4,150 of *****EARNED****** income, he can qualify as your dependent. The social security income he receives doesn't count for anything. Additionally, if social security was his only source of income in 2018, then he doesn't have to file a tax return at all.
