I've seen lots of people who can't initially sing in tune - and who've often been told to shut up in their school music lessons - be successfully taught relative pitch in adult life. They often become confident singers and are able to deliver a solo song well in public. It just takes a lot of dedication between student and a very skilled teacher. "Find you voice" type classes are available in many places here in the UK and some of them are very good - but it does rely on the calibre of the teacher.