A US Citizen currently living and working in Bangkok is moving back to the US to Atlanta, GA and will be needing immediate and on-going care for an accident in Bangkok, how can he sign up to receive immediate coverage upon arriving in the US? When arriving in the US, he will not be employed and will not have a ongoing income source. It is likely he will need long-term care.
When you move you qualify for special enrollment period, but if you have no income you should consider moving to a state that expanded Medicaid. Only states that expanded Medicaid will offer Medicaid to single adults with no income. Georgia did not expand Medicaid and you may find you have no coverage options. Check out what states expanded Medicaid here.
That being said, if you have household income, just not personal income, you can use special enrollment to enroll in a Marketplace plan. If you know you need expensive care, consider going with a Silver Marketplace plan due to the cost sharing reduction subsidies.