How can I get my son to be more outgoing

My son is an extremely shy person. He's about to be 17 and I'm worried he's not ready for the real world at all. He doesn't talk in school. His grade in U.S History is low because he doesn't participate. He doesn't get involved in class discussions or anything. I didn't know this, but for the longest time people in his school actually thought he was mute. He will not talk at all to anyone besides family really. He also tends to get a lot of uti 's because he doesn't even talk to ask to use the restroom and he won't use the bathroom if someone else is in the bathroom so he holds it for an extremely long period of time. I'm worried because I don't understand how he's gonna be able get a job one day if he's so scared to even speak. He has no friends or social skills. I'm just worried about his adult years. He wasn't always like this is the weird thing. He used to be a very outgoing kid until we went to live with my mom when he was 7. We stayed there for a year and a half an over time he got more and more quiet. There were like 7 other people living with so maybe he got over stimulated? I'm just not sure what to do to help him because he's just so shy and won't speak to anyone else besides us. Any advice with any parents who have shy kids?