Signed concerned sister. It is vitally important that your brother start HIV medications. HIV medications are the best assurance that his tumor will go away and stay away. All the suffering from the chemotherapy would be for nought if his tumor were to come back because he wasn't on HIV meds. You can tell that I feel pretty strongly about this.
Some regimens mix very well with chemotherapy.
Other regimens are also okay, but he should avoid AZT which does NOT mix well with chemotherapy. Also some of the protease inhibitors can be problematic in patients receiving CHOP chemotherapy. I have many many patients who were on CHOP chemotherapy plus anti-HIV drugs and did quite well. As for your second question about radiation therapy, side effects usually aren't too bad, but this depends upon exactly where the radiation is aimed at. I wish him luck.