We have a disabled (since birth) son who was receiving SSI until my husband took his Soc Sec early. My son is now receives 50% of my husband's full retirement benefit as a DAC. He's been offered an unexpected employment opportunity which would amount to more than twice his current Soc Sec benefit plus my son's benefit. If he takes that position, I understand he will be repaying all of his benefit as he is still two years away from his full retirement age. Question - Will SSA also have him repay the DAC benefit for my son and will that full repayment affect my son's eligibility of Medicaid and Medicare?
Unless your husband withdraws his claim, a return to work wouldn't necessarily mean that any benefits would need to be repaid. For example, if your husband started a job in January 2019 and notified Social Security of that fact, both his benefits and your son's benefits would likely need to be suspended depending on how much your husband will be earning. However, the benefits they received prior to 2019 would be unaffected by your husband's 2019 earnings.
That said, though, for any period of time that your husband's benefits need to be withheld due to his earnings, your son's benefits would also need to be withheld. And, if your husband ends up being overpaid for any past months due to his earnings or if he withdraws his claim altogether, no benefits would be payable to your son for the months that your husband wasn't due payments. In that case, Social Security would expect repayment of both of their benefits for the applicable months.
If your son does end up losing his disabled adult child's (DAC) benefits due to your husband's returning to work, he could reapply for SSI (Supplemental Security Income). But, there is no retroactivity allowed for SSI, so he couldn't be paid SSI benefits for any months prior to when he reapplies for them even if he ends up having to repay past months of DAC benefits.
Medicare eligibility doesn't start until the earlier of age 65 or the 25th month of entitlement to Social Security disability or DAC benefits. Whether or not your husband returns to work would not have any effect on your son's Medicare eligibility unless your husband withdraws his claim, which would also negate your son's DAC entitlement. Otherwise, your son would continue to be technically entitled to DAC even if his benefit payments are suspended due to your husband's earnings.
Medicaid is a needs based benefit independent from Social Security. My expertise is limited to Social Security benefits, so I'm unable to answer your question regarding Medicaid.