My boyfriend and I have been together 4 about 6months and every single time, he can't get hard enough? And I don know what is the problem? Could it be that he didn't have sex for a long time or what?

If he didn't have sex for a long time, he should have NO problems getting it up.

Has he masturbated prior to you and him getting intimate? Perhaps he's not aroused enough? It could be possibly a health issue. Can he get it up when he's by himself?

How old is he?

Yep. More info needed.

A few things that can hurt libido and erectile quality:
Diabetes
High blood pressure
Heart disease
Nervous system disease
Medications
Alcohol or drug abuse
Depression
Other physiological issues
Age
Fatigue
Etc!. there's a lot of possible answers, and many "wrong" ones for your specific situation.

So... it could be something as simple as nervousness, or mental hangups from past problems with sexual dysfunction or failure to please a previous partner, poor health to more serious issues...

Not enough info.

There are 100's of medical reasons that could be it, and other issues from watching too much porn, to past issues, He should have a physical exam
Also your age, for example if you were together the last 6 months in the local nursing home, it is a lot different than if he is 20.

This is going to be very blunt, but have you considered that you might be dating a head hunter? Simply put, next time you two are about to have sex, give him oral sex without him asking. Just go down, and see if that works. If not, he should see a doctor because there are many medical reasons that could be preventing full arousal, but I would try a different stimulation tactic first. Also, does he use any recreational drugs such as marijuana or steroids, because they have an affect on libido also.