Recently my lover has started to experience short term memory loss. There will be times when he loses his train of thought, or can't remember something that happened in the recent past. What could be causing this? Diet, fatigue, medication? Any suggestions on how to improve this - I know it is starting to concern him and I would like to be able to help him with this problem?

I am assuming your lover is HIV+. If so, it sounds like he may be developing HIV dementia. There are both clinical and radiologic (CT or MRI scan) ways to ascertain this. Often, memory will improve as part of an overall response to effective HIV therapy. There are also additional therapies that can be helpful to some people, such as treatment with Ritalin. Please urge him to be evaluated by his physician.