Dear motobare as far as anl sex is considered this is the most riskist activity without condom. But two points of hope in your case.
1-Your patner has undectable viral loud means that a chance of surviving but hiv is present in the blood.So you must go for HIV test.
2-As far as your concern of STD if your patner has not infected with any kind of STD than i do not think that any STD to you.
Just because your partner has an undetectable viral load he can still transmit HIV. His undetectable viral load is in his blood not his semen.
Unprotected anal sex is risky exposure to HIV. Your partner's undetectable viral load reduces your risk of HIV infection, but I'd recommend that you get tested to know your status for sure.