Visitor told me that she has the hiv virus but it is undetectable. my 7 year old son picked up a juice box and drank from it can he be infected?

Very unlikely: I'm not sure if you meant that the visitor was drinking from same juice box, but HIV transmission through such route is rare. Most transmission happen through exposure to infected blood. Outside healthcare facilities, most infections are through sexual contact and use of contaminated sharp objects like needles. If in doubt, talk to you doctor about screening test.