You need to get him out of your life now. Hes just looking for sex and he will never fall for you. He will use you and then flick you away like a used tissue and move onto someone else. Hes an asshole and even if his GF comes home and they get married-he will continue to cheat on her. You should try to find her on FB and send her an anonymous letter saying "your bf cheats on you all the time" -even give her the link to his profile online so she has proof and will hopefully dump him. He doesn't deserve her. It is cruel and wrong. Imagine marrying this guy and then finding out hes been cheating on you the whole time you were together before your wedding day. Its like marrying a stranger. Its all a lie.
Anyway they are two fools for even attempting long distance as it doesn't work but its not an excuse to cheat and lie. If he cant hack long distance-they should either agree to an open relationship or break up. What he is doing is wrong and don't feel sorry for him no matter what bullshit sop story he gives you.
Hes toxic and he will drag you down and break your heart so stay the f**k away from him. You sound like a smart girl who has good morals and strong integrity so don't be a fool for him and hold onto that good in you. Loyalty is a great quality to have as well as respecting others feelings and having emotional intelligence. Stay true to who you are and don't break your own rules for this asshole.
Good luck
"Don't ask a question if you can't handle the answer".