That is a very difficult situation.
The first step is he does absolutely need to get tested- It is always better to know than just to ignore. And if we diagnose someone as having HIV, the earlier they get on treatment, they usually do better than those that wait until they develop symptoms ( like weight loss or infections such as pneumonia or TB or meningitis) at an advanced stage of disease.
Seeing a doctor who is knowledgable about HIV , if he is infected, can tell him what the status of his immiune system is, and what medications he can/ should take.
As far as not having health coverage in Thailand- that is a more difficult issue. If he is there as a teacher, then I cannot see any way he would easily have access to health care in Thailand, other than paying by cash fee for service. I am not very familiar with their system but do know they have clinics and hospitals that cater to foreigners- and are quite good quality. But of course it would demand payment.
His best option may indeed be to return to his home country.
But again, getting tested is the first step.
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