Claiming him as a dependent would shift the penalty to your tax return. If he's on your return, then your income will make the determination as to whether you receive a penalty for having an uninsured dependent.

If there is a penalty on his return, that means his income isn't that low. In order to claim an unrelated person as a dependent, that person must have less than $4,050 in income. This is assuming he is not your foster child.(?) Though unrelated, a foster child counts as a qualifying child and therefore the income test doesn't apply. He must not, however, provide more than half of his own support.

See- Qualifying Child  vs Qualifying Relative

If he doesn't qualify as your dependent, did you look into the penalty exceptions for his return? One of them is unaffordable coverage.