As a teenager who's been in this kind of a relationship, I'd like to throw my two cents in. All this is based off of past experience, so it probably won't be exactly fitted to your scenario, though I'm sure you can get a few things out of it.
TL;DR Talk to him. Make sure he knows that you're there for him, and that you only want the best for him and his relationships.
Just FYI, I'm going to state everything as fact because it's much easier to provide advice that way. Let it be noted that everything except my personal experience is opinion.
She has visited our home about 5 times, and every time, she makes my husband and I feel uncomfortable. We find her to be very disrespectful and she behaves very sexually towards our son in front of us.
This is the biggest concern, the sexual behavior. While your statement that you can't cut off his contact with her is completely valid, you can put your foot down and refuse to allow her in the house. If you have good reason to keep her out of your home (which it seems you do), do it.
The girlfriend tries to manipulate our son by crying and hyperventilating in order to force our hand (to allow them to meet up as well as many other things.) She often upsets him saying she is going to go overseas without him etc..He just can not see logic when she plays these games.
This seems like a typical manipulative girlfriend, and a boy who's simply convinced that he's in love with her. IMHO, the girl doesn't seem like a great person. It's possible that she's simply going through struggles in her life, and will get over her behavior, but for now, their relationship isn't very healthy (as you've noticed). I think a great thing you could do for your son is to sit down and have a long talk with him about... life. Girls and relationships in general make for a good, long, heart-to-heart talk. You could take this approach, with emphasis on his current one, and how she's not good for him.
A couple of points you might want to include are:
- She may only be using him for his body. (This is what happened to me - I was sure I loved a girl, and sure she loved me, and it turned out... she just wanted sex).
- He may just be telling himself that he's in love. A young man will often do this - when a girl is attractive, or attracted to him, he will easily overlook her obviously negative qualities, things that would not make a good life partner.
He has become moody, secretive, won't do his chores, and his grades have slipped.
Well, this is typical for teenage relationships, especially first ones. Moodiness and secretiveness are to be expected from a teenager in general, so it's not too big of a problem. If it concerns you enough or seems to be a big enough deal, you could try to assure him you'll listen to him and help him through anything he needs.
Not doing chores may be a sign that he isn't feeling love or support from you - and you're not in the wrong to withhold support for his relationship. He needs to realize that you know and understand what he's going through. As I previously stated, have a heart-to-heart talk with him. But you'd do well to make sure it doesn't turn into a lecture; teens (*cough*) don't take lectures well.
The slipping grades are a sign that he's talking to her too much, and in the place of his studies. Members of the opposite gender* are distracting in general, but when a teenager is in a relationship, it's even harder to focus on the important things in life.
*Please don't hate on me for gender inequality. I wasn't being insensitive to any types of people; I'm simply stating my opinions and views on the matter.
I'm not sure what you can do for him in this case. Teenagers are exceptionally stubborn about relationships (which you'll find out soon enough, and you probably experienced as a youth). If he cares more about texting her than about keeping up his grades... you might want to try punishment*.
*I know this isn't my place; it's just a suggestion.
We really don't know how to handle this obsession as it appears to have become.
There are three things that will probably happen:
- He'll realize on his own that the relationship is unhealthy, and break it off.
- He'll eventually listen to your judgement after being internally conflicted for a bit.
- She'll do something hurtful to him, and he'll end it.
I can (most likely) assure you that this relationship will not last; you don't have to worry about that. If talking doesn't work, just wait. It may take months or even years, but it will die. Your son will come off better because of it; he'll realize what he actually wants in a partner, and he probably won't make the same mistake again.
I'd say it's imperative that while giving him the due amount of privacy a teenager should have, make sure he knows that he can always come to talk to you about anything.
Hope this helps.