Well she is showing she is immature and does not have the experience of a mature relationship.
They have only been dating 4 weeks. THAT says it all... this is not something to determine his life. 4 weeks is nothing... (not trying to be insensitive), but it is.
he is so young... sure old enough to join the Marines... but so young in terms of love/relationships/knowing what kind of "woman" he wants to spend his life with etc. He should not give up on his "dreams" either... but yes, being a gentleman if they break-up and it does not work out.
And, her giving him that "ultimatum" is well, just childish. It is emotional manipulation.
As a person ages, and grows up, their ideals and ideas about relationships ALSO changes... as THEY mature. Right now, after only 4 weeks... that is really not anything to really bank on.
Sure, leaving anyone hanging is hard... but in reality, that is life and sometimes there does need to be reflection upon a relationship... and not making just spontaneous decisions. A MATURE "adult" would understand that, albeit hard emotionally.
A 4 week relationship, is a shot in the dark. It has not even evolved, to where they can predict their relationship OR compatibility, wisely.
True, your son does NOT know where their relationship will be then.. and he is right. Graduation is important... as well as his aspirations. A supportive and mature and mature love relationship... will understand that. Your son, is considering very real options... for his life. And he seems to be thinking about it, clearly, maturely. BUT... he does need to ascertain the difference between a fickle love/relationship and one that has gone the distance despite hardship. THIS relationship, has not. It has only been 4 weeks.
He needs to be honest with her. Even if it is hard. Even if it breaks up.
You are not wrong. He does need to learn all he can... to be successful in life... and to be wise. This includes his learning about relationships. And, as young as he is, his ideas about relationships will most probably change. A 17 year old, and a 27 year old, and a 37 year old... does NOT have the same feelings/ideas about relationships... because they mature and learn what they want and don't want in a partner....and how to conduct a relationship and evaluate it, and what they experience in life.
He needs to think about that.
Make sure, the girl does not try and get pregnant, to try and keep him.
She sounds immature. I don't mean to be insulting... but is she is only 17 as well... she won't have mature judgment about it, nor him, nor their relationship.
Guide him... just be there for him.
Just some quick thoughts, and from what I"ve been through in life.
All the best,
Susan