My 8yr. Old son had been vomiting from stomach virus done with that but he cries when he has to walk said right leg & lower stomach hurt?

Leg painPOSTvomiting. Your son must have had a lot of vomiting and lost electrolytes due to that. Low potassium can cause muscle cramps. His lower stomach probably still hurts due to the inflammation form the infection and not having had much od in his stomach. Try giving him banana and see if it helps. If it continues, you should see his pediatrician.
M.D. visit. After a vomiting illness the muscles of the abdomen and chest may be strained and cause pain. The pain that is caused or referred by moving though is of concern for inflammation of the lining of the abdomen or another cause unrelated to the original illness. I would suggest following up with your pediatrician, keeping him well hydrated, providing yogurt daily and checking his temperature.