My son was arrested for domestic assault but case was dissmissed in pre trial. ?

May not be a problem since the case was dismissed, but be careful not to repeat the scene !

Even though the case was dismissed, his arrest and case info is still viewable on his record as dismissed. In order to have his record cleaned, it must be expunged through the police department... not the court.

EDIT: Can his current status effect employment or getting an apartment? It may, because it was a case involving assault and it's all about perception of character. However, because of the dismissal, his chances are better because he was never actually convicted of the crime.

Depends on the application process. He'll have to say he was "arrested," but he can say he wasn't "convicted." A conviction is far more derogatory than an arrest. A few jobs look for financial security, so domestic violence may not disqualify him. A few jobs require a squeaky clean record, so he might not get that job. Sure, it would have been better to not get arrested, but he hasn't damaged himself fatally. ... Good luck!

How was it dismissed? Did he get one of those deals where the case is dismissed, but he has it on his record that he did a plea bargain kind of deal?

If so, he won't have to say he's been convicted of a crime, but on many applications (not really for housing) he'll have to come clean with the fact that he's accepted a plea deal.

If he can keep his nose clean for 5-7 years he may be able to get his record expunged

No, only if he was convicted would it show up in his criminal record.

And besides, it's misdemeanor.