In Pattaya I've often heard of Dr Olivier being mentioned for HIV and sex-health related issues, gay and straight. He should be your first stop. I'm quite sure I read somewhere (Gaybuttonthai?) that he has treated Thai guys with HIV.
Google "Dr Olivier Clinic Pattaya".
The HIV doctor is Dr. Phillippe from HEARTT 2000, he has been treating the boys and girls of Pattaya for at least 20 years now