What medication is your son taking? Some meds are longer-acting than others. Lamictal is one example. Being a few hours late with Lamictal isn't as big a deal for me than it was with say, Trileptal. A missed dose of Trileptal and I definitely felt it by evening. Lamictal, I've missed a whole dose and been okay.
The dosage matters, too. If it's just barely at the level to control his seizures and he misses a dose....
A good way to see how fast his medication may clear out of his body is to look at the half-life. That's the time it takes for the level of medication in his system to lower by half (or in other words the time it takes half the drug to be eliminated by his system). Sometimes the doctor has people take their seizure meds 3x/day - this is because that medication has a short half-life and taking it more often helps avoid fluctuation of medication levels in the blood. The longer the half-life the more likely it is that he could miss a dose and not have it affect him as much.
Online it looks like carbatrol has a variable half-life, in some cases 12-17 hours or 30-40 hours. I don't understand the difference. Maybe you can make heads or tails of it?
http://www.drugs.com/pro/carbatrol.html
For comparison's sake, Lamotragine has what's considered a long half-life of around 32 hours and Trileptal's is relatively short - around 8-10 hours.
http://stanfordhospital.org/clinics...lepsy/medicationTherapy/medOxcarbazepine.html
http://www.drugs.com/pro/lamotrigine.html
Also sometimes people's bodies become accustomed to their medication over time and they need the dosage increased, or their bodies or seizures change which also needs a med change.
Q: is there a reason your son is missing doses? Does he refuse to take them? Or is the family forgetting to give them to him? There are a lot of great reminder tools ranging from simple alarms to programs that will automatically call you on the phone to pill bottles that hang around your neck and beep. I always keep an extra supply of pills either in my pocket or purse. They have little air and water-tight pill bottles at the drug store. If I forget a dose I can take it right away no matter where I am.
Be sure to call his neurologist and describe what happened. The neuro will know what to do.