I saw your question posted on Sunday April 29 2012, only now ( Friday June 29th Indian standard Time).
Though it will not be useful to you, since this is in public forum, some aspect of the following answer will be useful to few parents and grand parents.
Let me first salute your affection for your grandson. This is probably the greatest gift you can give to your daughter, by taking care of her son.
In general let us follow this rule. If as a caretaker, something tells you that all is not well, take a medical opinion
Yes. Viral fever
can cause these symptoms and can make the child lethargic.
In such doubtful situations apply these four simple steps
1. How is his sleep. Is he able to sleep undisturbed?
2. How is his fluid intake. Even if he is not able to consume solids, his fluid intake should be fair
3. How is activity: When he is awake is he oriented? Recognize you as grand parent, he knows where he is and can roughly tell which time of the day it is.
4. Fever level: More than 104 F is high.
1. How is his urine? He has to void atleast 3 times to 4 times in a day
2. How was his motion? Was there any blood?
3. Is there any red or purple patches in the skin ? Indicates serious viral illness or complication
4. Did he have any yellow or green colour vomiting? Indicates stomach ulceration, again pointer to careful observation
1. How are his eyes? any evidence of jaundice?
2. How is mouth ? any painful ulceration?
3. How is neck? Any swelling or stiffness? swelling can be due to infection, stiffness may be a pointer to problems inside the skull
4. How are the limbs? Are they swollen
1. Any ear discharge
2. Any nose leak or nasal tip redness
3. Any Fast breathing
Above problems can be evidence of bacterial infection
4. Severe cough? Post viral : cough can be a problem
Hope he should be doing fine.
If you are worried any time, always consult. We doctors, respect and understand your anxiety