Welcome to the forum. Thanks for your question.
This was essentially a zero risk exposure for HIV. Most people don't lie when asked directly about HIV status; and even if your partner was infected, oral sex very rarely transmits HIV -- many experts consider it entirely risk free, especially for the penile partner. For a detailed discussion about oral sex and HIV, see the thread linked below.
The risk of some STDs is a bit higher -- low for all (e.g., gonorrhea, syphilis) and virtually zero for others (e.g., chlamydia).
Your med student friend has some learning to do about STDs. I wouldn't have recommended any antibiotic treatment at all; and amoxicillin is a poor choice for STD prevention.
What to do now? The risk of any STD was sufficiently low that I really don't recommend anything, assuming you don't develop symptoms like penile blsiters or open sores, or discharge of pus or mucus from your penis. However, if you want additional reassurance, you could see a doctor or clinic (e.g. your local health department STD clinic) for an genital area examination and testing for gonorrhea, chlamydia, and HIV. (There is no point in syphilis testing -- that's the one STD that would have been well prevented by amoxicillin, so no point in testing.)
I hope this has helped. Best wishes-- HHH, MD