my son was sitting in drver position keys in glove box had been drinking can he be charged with drunk driving if he was not seen driving?

Even though the keys were not in the ignition, typically if a driver as the ability to exert control over the vehicle, that will be enough.  So if the keys were somewhere else in the vehicle (here the glove box), that would count as "driving".  As for the arresting officer not seeing this, officers usually need not observe someone driving in order to arrest them for drunk driving.  Circumstantial evidence of driving is typically sufficient to establish this element.

However, this is not to say that there has not been a procedural impropriety.  This is a very technical area of the law and an experienced DUI attorney could possibly use this, or even something else you haven't thought of, to get the charge dismissed or even reduced.

If this is your son's second DUI in 2 years it will be well worth it it hire an attorney to represent him.  Even if there is a conviction a lawyer can be instrumental in getting a lesser penalty imposed.