Post rta patnt acdntly daignsd as HIV+

My cousin had a RTA few days ago. He was first taken the hospital where he was given first aid and tetanus toxoid. But later that night he had pain in the abdomen and immediately again taken to hospital. On workup he had grade 3 spleen trauma and was managed conservatively. But was asked for preoperative tests. Just in case if the conservative management doesnt work. The real problem started when after doing triple serology, he was said to have HIV POSITIVE. We were shocked at first but then thought there can be some mistake. He is stable right now but is very worried about that HIV POSITIVE. I have few questions 1) whats the further workup to be sure? 2) can the test be false positive? He didnthave any symptoms before this of any kind. Completely fine young man. And belongs to a conservative society. The family is really confused now and really wants to know what can be done further.