How to know if medication is working?

Hello all. I have been so confused and full of anxiety the past few months and hoping to get some feedback/ideas. My 16 year old son has had epilepsy since age 5. Idiopathic, all during sleep except one when he was weaned off to see if he'd outgrown it, normal MRI and EEG's show sporadic and rare inter-ictal discharges and spikes. He was very well controlled with Trileptal for years, had some random breakthroughs when he hit puberty and as dose was increased to accommodate growth the side effects became intolerable. This past October he had 3 TC's in one day due to his sodium levels dipping to low due too the Trileptal, at which point his neurologist started making the transition to Keppra, with the goal being monotherapy. Since then he's had breakthroughs 3 different times, all seemingly due to medications levels. The first breakthrough was 2 weeks into the conversion and occurred a day after dropping the Trileptal for the third time in 10 days (he was originally at 900 2X a day and within 10 days dropped to 150 2X a day). Doc put him back up to 300 2X a day Trileptal and Keppra was titrated up to 1500 2X a day (he weighs about 185 pounds). After about 3 weeks the Trileptal was VERY SLOWLY reduced over the course of several months. The next breakthrough happened 11 weeks later, right after the pharmacy switched manufacturers of the generic Keppra he'd been taking. Doc gave authorization for brand name only and raised him to 1750 am, 1500 PM Keppra. Things were good for a month. He has still been slowly weaning off the Trileptal. Last week, after 2 weeks of being on 75 mg AM and PM I dropped it to zero PM and 75 AM. Wouldn't you know, the next day during a nap after school he had a breakthrough! So frustrating! So doc upped the Keppra to 1750 2X a day and I put the Trileptal back to 75mg 2X a day. It's only been 3 days but so far so good. We will get a Keppra blood level this week, his last level was not where the doc wanted it to be, he seems to metabolize the Keppra very quickly because at 3000 mg per day his peak level was 25. Doc wants to see his trough level at 25. Anyway my confusion is this: If the slight change in generics and the slight drop in Trileptal both triggered breakthroughs, is the Keppra doing anything at all? I thought the whole purpose of the drug was to stop seizures from triggers that would normally cause them. He never had frequent seizures even when unmedicated, so could it be that the Keppra is doing nothing at all? I am just so beside myself with worry that he will catch the illnesses that are going around the schools (Strep, or the flu) and have even more seizures. Because if Keppra didn't protect him from the slight blood drop in Trilptal, how will it protect him from any trigger? Am I making sense? The doctor's answer is always "Maybe he just needs more Keppra to get a theraputic level". But Lord, how much Keppra can we give the kid, he's already over the recommended top dose! As much as I dread the idea of switching meds again I just feel like maybe the Keppra isn't working. We have an appointment with a great doctor at the NYU Langone Epilepsy Center in April. God I wish it was sooner! I know I've rambled on but I just have this mass of anxiety sitting in my chest, I want to make sure he's getting the best treatment and lately I feel like it's all a big crapshoot. So I guess my question is this: Can an AED still protect against other triggers even though it doesn't protect against drops in blood levels of medications? Any words of wisdom are welcomed. I am so sad for my son. :(