I am sorry for the delay, but I had stepped away from the computer.
The blood tests would be able to say whether he has previously been infected with HSV1, HSV2, both, or nether. But in someone with no symptoms, if either HSV1 or HSV2 is positive, it will not be able to say the location of the infection.
If your concern is only about HSV2, then this encounter entailed minimal risk, since the greater risk would be the transmission of HSV1 to the genitals via oral sex. Certainly, the prostitute is at risk for a genital HSV2 infection, but it would typically be a genital infection and would not have been transmitted from this encounter of oral sex. However, if either you or he was ever sexually active engaging in penile-vaginal sex before your current relationship, then either of you could have gotten infected with HSV2, and the only way to be certain that either of you are not infected would be to check a blood test.
If both of you have never engaged in penile-vaginal sex before your current relationship, then there is no reason to be concerned about having HSV2 from this single encounter.
However, that is a different issue than concern over whether he may have gotten infected during this encounter of oral sex with a prostitute, in which the risk is a genital infection of HSV1. If he is negative for both HSV1 and HSV2, then this would be excluded, but it is statistically more likely that he will be positive for HSV1, because most people are positive, and the blood tests will not help you if that happens.