My son receives SSI which is barely a subsistance amount. He does not owe current child support, but has an arrearage amount. In the state of Oregon, your driver's license can be lifted if your support payments get behind more than $2500.00. My question is whether that applies if your only income is from SSI. He cannot afford the back child support payments, but he also cannot afford to lose his driver's license. He lives in a rural area that does not have public transporttion, and needs to be able to drive to doctors appointments, etc.

Why did he get behind is the issue, it should have been set on his income at the time, if his income reduced he would have applied to modify the amount to pay.
They can not garnish the payment but it does not mean support is not owed.

But yes they may suspend his license till he makes the payments or has court approved payment plan. Have you considered helping him with it ?