Is it normal for a Mom to keep a 19 year old boy trapped at home like a caged animal?

My MIL isn't normal at all. She doesn't associate with anyone. She is in her mid 40's. She has 4 kids. My husband has a strained relationship with her so does his older sis. She still has 2 kids at home one 17 and one 19. These poor kids have no social life at all. The son has always been home schooled (She claims his asthma has always been too severe for school). The daughter does go to public school but that's her only escape to the REAL world. The son is only allowed to go outside in the woods to hunt and an occasional ride with my husband. She goes all ballistic and makes him come right back home after about an hour. The son has expressed to my husband recently that he is tired of this. He says that he befriended some neighborhood kids and they sometimes visit him. He said he catches his Mom with the windows up listening to his convos. She will also barge in his room when she hears him on his cell and asks him who he's talking to and about what. He is quite embarassed about how she treats him. He enrolled in college but I don't think he still attends. His Mom has drilled in his head that his asthma is too bad. She was taking him to college and picking him up daily. He has driver license and a car but she won't allow him to drive.
This kid is stuck between a rock and a hard place. I don't know how severe his asthma is, all I know is the times I've seen him he hasn't had an attack. He keeps his meds with him and a portable machine. I'm guessing he won't be able to work with his asthma though he wants to cause he wants out of there. We have a full house we can't take him in. I'm just afraid this kid will run off and my Hubby will never see his little brother again. This will be the second sibling to do this as he hasn't seen his older sis in about 12 years. I never say anything I just listen to my husband as he talks about it. I don't know what to tell them.