My non-disabled son receives child insurance benefits on my record. I receive SSDI. Is his benefit subject to an annual earnings limitation based on his own earnings?
1801.3 (b) at the following link says that beneficiaries are not subject to an earnings test if they are entitled to benefits based on a disability. This is not clear to me if it means "their" disability or my disability. It seems odd to say they are not subject to an earnings test if they are disabled, as if they were entitled to benefits based on their disability, then it would make more sense that they be subject to an earnings test. See second link pertaining to substantial gainful activity.
Yes, if your son is not disabled and he's receiving child's benefits because he is under age 18 or age 18 to 19 and still in high school, then he would be subject to the regular Social Security earnings test. That's the test referred to in section 1801 of Social Security's handbook, which is the first link that you posted in your question. Your son could earn up to $17,640 in 2019 without losing any benefits, but he'd lose $1 of benefits for each $2 that he earns in excess of that amount.
Individuals who are disabled and receiving either Social Security disability (SSDI) benefits, disabled adult child's benefits or disabled widow's benefits are not subject to the regular Social Security earnings test, but are instead subject to a different work test. Disabled beneficiaries generally get a one-time 9 month trial work period, after which their benefits could be terminated if they earn more than the substantial gainful activity (SGA) limit. The monthly SGA limit for 2019 is $1220. That's the test referred to in the second link posted in your question, and it's the test that would apply to you if you work while receiving SSDI benefits.