I can relate!! I have a friend whose 13 year old son is exactly the same. Always hugging and kissing her and holding her hand during movies. I found it very strange and uncomfortable. He's been diagnosed with autism and aspergers and I feel like he's way too dependent on her. He doesn't really have any other friends and is very anti-social. She takes him everywhere and I think she's happy to have someone to do things with all the time. But I think it's very unhealthy and she should be encouraging him to make some friends.
Most likey he don't have a daddy in his life or he is just very spoiled and yes she will get defensive that is a big part of the problem in it's self.
I have a son who is now 18 years old, but when he was 14 he was the same way, he still called my mommy and would come and lay on me when I was sitting at the tv, or come up and rub my back when I was cooking etc. It seemed like a lot of dependency and he wouldn't go to a counselor so I did, and it had a lot to do with the seperation from his father. He was confused as to his role after the father left. This lasted until my son was about 16.5 and slowly got better. I no longer tuck him in and he no longer sits half on me. It takes some time and mom getting a little tougher on him acting like a baby. Unfortunately my son and I ended up having a big argument at first and he was upset and wouldn't talk to me for a couple of weeks, and then we had to do some rebuilding.
I am very sure that my son who now has friends his own age, and his first girlfriend that he is far from gay. I would bet that this boy too just needs some boundary's and maybe some counseling (I chose not to push counselling with my son, but would push if I had it to do over). Good luck, it is a confusing time for boys.
It sounds as though your girl friends son has high levels of anxiety. Mum is his security blanket and anything outside of her, he's unsure about and probably scares him. As you you probably know, as soon as kids become teenagers, they begin to want alot more space away from their parents, don't want to be seen around parents, and instead be around their friends. It's possible he has problems socialising or making friends (lack of confidence?) and maybe this makes school hard for him?
Obviously she has to start encouraging him to be more independant, not tucking him in of a night etc. it might be worth seeing what's going on at school, if there's any problems there too. I don't think he's gay, i think he's just immature for his age, and the actions/way he talks aren't age approprite for his age, maybe this is why people/family thinks he's gay. Plus i don't think that's really important to focus on at the moment, as he'll work it his sexuality in time. good luck :)