Help me -- my son has become someone else

My almost 8yo son was diagnosed with complex partial seizures this past January. He was on trileptal at first, but had more seizures than ever. We changed medications to Keppra (and changed doctors as well!) in June, and the seizures have improved (I guess that's the way to put it...). However, he seems depressed; he has become hostile and aggressive; and he has some anger management issues. We are so new to this -- we didn't know if this is just an age "thing" or if it is the meds. His twin is experiencing some of this attitudinal stuff, but in a different way -- their personalities are opposites to begin with -- which is why we wondered if it was the age. After reading lots of posts on this website, I am sitting here in tears, wondering what this disease is doing to my son's brain; his once-positive, sweet, helpful, compassionate nature; and to our family. How do I handle this? What do I tell the dr.? How do I discipline, while still giving allowances for the meds, but without making this a crutch or an excuse for horrible behavior? Any advice would be extremely helpful. Thanks. Kim