I am sorry but this story sounds implausible. Attacked by needle poke or HIV phobia? I'm going with an anxiety disorder here.
Regardless, playing along although I do know better than feeding into anxiety, but people get HIV from having unprotected vaginal or anal sex or sharing IV drug needles. NOT getting stabbed to them but INJECTING drugs. So, maybe your anxiety loop will stop when you take that into account but I am pretty sure your son was not stabbed with a needle on a casual walk.