First off for his food allergies the Eagle pak's Natural would not be my first choice because in addition to lamb and rice it also contains chicken
, egg and corn, all of which are common allergens. I would switch him to Science Diet Z/D or D/D (prescription from you vet), or a second best choice would be California Naturals Lamb and Rice or Natural Balance Venison and sweet potato.
I don't know what does of fish oil you give, but I find many people do not give enough. To really be effective with skin issues 1000 mg per 10 lbs per day is needed. I have not found Magnesium to be helpful for allergies.
As for the seizure. Magnesium can help a little for epilepsy
, but you will need your vet to rule out other causes of seizures first with an exam and blood work. If no other cause is found then usually epilepsy is not treated based upon one seizure, but treatment is started when the patient has either more then one seizure a month, several in a row or one that lasts more then 10 minutes. If that is occurring there are other medications that are much more effective then magnesium such as Potassium Bromide or Phenobarbital.
Regardless, if you want to try magnesium anyway the dose is 1-2 mEq/kg/day. Beware it can cause diarrhea and should not be given if you dog has a history of bladder stones.