It is VERY unusual to prescribe Keppra after such a short history. I would seriously question that decision. Seizures tend to not come on suddenly in this way without an identifiable external cause, and normally a physician will seek out this cause. I've seen, increasingly, some doctors prescribing Keppra almost prophylactically just to see if the patient responds. The problem with this approach is that, with such a short history, it is impossible to know if or how well the patient responded to the drug therapy.
Knowing only what you wrote, this is a very questionable call on the doctor's part. Of course, there may well be more to it, of which I am unaware. Still, I would press the doctor on this.
As for the military, I spent 10 years in and have some experience with medical discharges (though I did not seek or get one myself). I can tell you that the military will not allow your husband to remain in the military with uncontrolled seizures - "uncontrolled" being the key word. If the seizures can be controlled, then it becomes less cut-and-dried.
Disability is measured as a percentage, and this is judged by a review board. This percentage determines the pay and benefits to which you are entitled. There is a cutoff percentage, above which one is entitled to certain benefits such as a free bachelors degree and other education support. As I recall that is only 10% (or at least it used to be.) The percentage also determines the size of the disability check.
If the military forces your husband to leave the military due to seizures, then he will almost certainly get at least 10%. One important thing though, seizures are a little funny because they can come and go. If he started having seizures suddenly and without a known cause, then a review board will rightfully conclude that the condition may be temporary. This can affect benefits, and can even prevent granting a medical discharge in the first place. In any case, should the seizures resolve in the future, this will make him ineligible for benefits.
I would proceed carefully. Seizures are an easy illness to fake, and so, some people have tried this as a ploy to get a medical discharge and benefits. The problem is that, as a result, seizures are met with more scrutiny, particularly if no external cause or evidence (such as a positive EEG) can corroborate the medical claim.
As you go down this road, make sure that you exhaust treatment options and attempt to stay in the military rather than actively seek a medical discharge. That way, if the seizures do indeed resolve, then his earning potential is not disrupted and your medical expenses are covered (through the expensive diagnostic phase of treatment). Make the military force him to leave if he is unable to safely do his job. On the other side of that coin, make sure your doctor is granting appropriate profiles to keep your husband from potentially unsafe activities. (For you non-vet types, a "profile" is a sort of prescription that a doctor writes to indicate in what ways your duty is restricted due to medical issues.)
