The only person who would know if he can increase his dosage is his doctor. My understanding is that there has to be certain antibodies present in his bloodstream, that can only be seen during a blood test. I know that seizure meds can have negative side effects. You can have memory loss, shakes, but I can't know for sure if that is what he is feeling. I am not a doctor just a person who knows a thing or two about this. Please come back and update us on how he is doing.
Your husband's Dilantin level may be low due to the way his body is using it.The med is known to 'find it's way' to all the organs,the bones and teeth.I was recently talking to someone who yrs ago had the same problem with a low Dilantin count.Her Dr even accused her of not taking her medication! But when she ended up in the hospital and was being given the medicine,that arguement would no longer suffice! Testing was done and she was told the med was "leaking" into other places.So your husband definitely needs to see his Dr! No home-doctoring!