I know this is not going to be popular, but if he is really on antivirals then the risk of transmission is incredibly low. According to the "Canadian consensus statement on HIV and its transmission in the context of criminal law" (put together by experts on HIV):

• Where the HIV-positive individual is on effective antiretroviral therapy, anal-penile intercourse likely poses a negligible possibility of transmitting HIV even in the absence of condom use.
• Oral sex performed by an HIV-positive individual on an HIV-negative individual poses no possibility of transmitting HIV.
• Where a condom is used or the HIV-positive individual is on effective antiretroviral therapy, oral sex performed on a HIV-positive individual poses a negligible possibility of transmitting HIV.

What does negligible or no possibility mean?

• Negligible possibility: The basic conditions of viral transmission are potentially present. Isolated reports of transmission have been linked to some of these activities, although they have been difficult to confirm. The efficiency of transmission appears to be negligible and transmission is highly unlikely, if not impossible in most circumstances.
• No possibility: The basic conditions of viral transmission are not present. No occurrence of transmission has been reported. The virus is not transmitted by these activities

In other words, this is not really Russian roulette. Unless your theoretical gun has a single bullet amidst thousands upon thousands of empty chambers.

I'm not saying Robert is right in not telling Diego -- in any trusting relationship you should not keep things like this a secret. And just by reading this thread, if he is like most people he would be horrified at having this kept from him. This indeed will likely end their relationship. But realistically, if you at all understand science and statistics, this is really a minuscule issue in terms of risk and thus it shouldn't be framed as such.