Retarded Ejaculation
There are many reasons for "retarded ejaculation," mostly having origins in psychological factors. The two most common reasons for this condition are fears of impregnating a woman and the need to control the level of intimacy; both can be simultaneously present. The former reason is self-explanatory; the latter is more complicated. Many men have conscious or unconscious attitudes about intimacy and sex. They can either be emotionally intimate or sexually intimate but not both with the same woman. These men are often capable of climaxing with prostitutes, but not with a partner with whom they are intimately attached. Since you report that he has not been able to ejaculate even when participating in anonymous or non-intimate sex with "sex workers," it appears that the issue may be related to the former issue. Without having direct contact with him for a detailed exploration of his psychological and sex history, it is not possible for me to determine etiology.
That being said, I can suggest that your partner has been training himself for many years not to ejaculate inside of a woman, in effect practicing a form of birth control. Now with being HIV positive, he has the added fear of transmitting the virus. This would be sufficient to create a problem for most men. In this regard, I would strongly suggest that he find a professional psychologist to talk with in order to help deal with the psychological implications of being HIV positive, i.e., what it means to him to be HIV positive. I would also be interested in knowing how he contracted the virus. If he contracted it through sexual intercourse, for example, this could serve to confirm his unconscious beliefs in the dangers of sexual intercourse and ejaculation.
There are several things that the two of you can practice that might help alleviate the situation, but there are no guarantees. Since you report that he is able to achieve orgasm through masturbation with the aid of pornography, it appears that he has trained himself to be sexually aroused to hardcore, sexually explicit material rather than through emotionally connected intimacy. He has separated sexuality from intimacy, relegating sexuality to mere release. Continuing this practice of isolating sexuality to self-stimulation exacerbates the problem. What is necessary to bring a live partner -- namely you -- with whom he is emotionally connected into his sexual activity thereby transferring the sexual experience to interactive sexuality.
So for starters you might begin by watching some pornography together to promote a sexual response while touching and caressing each other. What we would want to do here is eventually to have you bring him to climax manually rather than him doing it to himself. The key, however, is to connect the sexual response to an overall emotional and physical connection with you rather than to the pornography. Gradually, as success is achieved in this manner with you bringing him to climax (beginning with both of you fondling him so that he can show you how he likes to have it done), you can attempt penetration even if it requires disengagement to complete the ejaculatory process manually. With practice in graduated increments a complete transfer might be achieved. I say "might be achieved" because there are so many complications involved: the number of years that he has practicing his own form of sexual response, the HIV, using a condom which decreases sensitivity, and whatever other psychological factors might be in play.