I would be worried about the facebook issue, especially when he changed his profile pic to himself and this other girl.. you could always ask him for his passwords. my friend had a simular issue and her boyfriend gave her his password and she read all the messages from his other girl and could see that he did tell her to go away.
in regards to the visa, i think you need to double check why it was rejected cos they dont need to be in the same job or a year. my friends boyfriend got a visa and he hadn't even got a job at the time he applied for the visa and didnt have one to go back to.
my boyfriend has got a job and is applying now but cos they work the season over their its very rare for them to have the same job for a whole year.
they do need to show they have an income too though... he wont be able to come to england to study, its hard enough getting them over just for a visit nevermind to study. Also, given that you're only 17 wont go in your favour cos you have to prove that you can accomodate them whilst they are over here, so if you dont have your own place and income it will be difficult. not impossible but you will have a battle on your hands hun. you would have to show that your parents are willing to accomodate him and prove that you live with them etc.
in regards to the thomas cook thing about booking a flight, you are best checking with them directly,
i have flown out by myself to meet my boyfriend but have had friends in turkey so i feel safe and i am a little older than you hun.
good luck xx