After the respiratory and diarrheal episode- how has been his food and fluid intake?
Usually after severe viral illnesses- such weakness is seen for a couple of weeks.
You have to be sure that he drinks well.
Give him plenty of fluids- fresh fruit juices, vegetable soups, milk, porridge, curd etc.
Be sure he passes urine at least 4 to 5 times in a day.
Give him food items which he likes.
Since you say his blood tests were all normal- I guess there is nothing much to worry.
Give him bed rest
for a week and give him nutritious diet- he should be okay with this.
However for your reassurance, you may visit the Pediatric Hospital and get him examined thoroughly.
Hope my answer helps you.
Wish your son good health.