My 24 mth sons eyes roll in the back of his head when he is tired should I be worried about seizures had a negative 20 min EEG when he was 1?

Eye roll is normal. Our eyes automatically roll up when we blink, and when we are tired. This is more common in children where they often keep the eyes open. The eyes can drift up and out and then snap back to normal if altered. There is no reason to suspect seizures.
Probably not. This may just be something he does. If he is not non-responsive, twitching or turning blue with no loss of bladder or bowel control and no post episode confusion then these are probably not seizures. It would be wise to see a pedi-neurologist to verify that this is the case.
Likely normal. Wthe eyes normally roll upwards when closed, due to a protective reflex known as bells phenomenon. This can be seen in people whose eyelids do not close completely. In isolation (without other symptoms), eye rolling such as this is normal and is not a sign of seizure.