I hope your son starts eating like an 11 year old boy soon. It is very frustrating and painful to see your child not eat or struggle to eat.
Since there is dysphagia to solids only, either there is mechanical obstruction to food or this is functional. It could have progressed because of the energy drinks too because your son’s calorie requirement and thus hunger are satisfied. It is less likely to be a motility issue as this would affect both solids and liquids.
Another possibility is a bad gastric reflux. Now, mechanical obstructions like strictures are visible on barium meal. Endoscopy is able to diagnose a gastric reflux or webs and other mechanical obstructions such as tumors or rings due to eosinophilic esophagitis. You need to thus find out what the endoscopy result says. The biopsy results may take time to come, but the doctor may have found a region of redness or reflux or something abnormal and picked up the tissue from this region to biopsy. Please discuss this with your doctor.
Also, instead of energy drinks, try soups, custard, pudding, egg mixed with milk, milk shakes with fruits, milk shakes with dry fruits, fresh fruit juice, smoothies, and other semisolid dishes. This will provide the nutrients and build up his appetite as well. With time he may be able to eat better. Also, it will end his struggle with food. The more time a child spends trying to eat, the more difficult it becomes for the child to eat.
Keep his favorite biscuits/cookies/cakes/food items on table. Do not ask him to eat. Engage him into active sports like football, swimming etc or going out to play with friends. Usually just after sports a child is hungry and usually grabs food. Also, try and not think much about his eating. Just change the food preparations such that they are easy to swallow.
I am not saying that he has functional dysphagia. But all dysphagias do have a functional component. So, all the above mentioned measures will take care of that.
Less common causes of dysphagia to solids is an enlarged heart, a tumor in the chest, enlarged aorta, projecting superior cornu of the thyroid cartilage, and fungal infection of esophagus. Another possibility is irritation of the anterior fibers of C2-3 spinal nerves with spasm of the muscles attached to hyoid bone. This usually happens due to injury or postural problems causing curving of lower cervical spine. If the measures I suggested do not help, and if the endoscopy report is normal, then discuss these possibilities with the doctor.
I sincerely hope you will find this information useful in your journey towards better health for your son. Good Luck and take care!