I am sorry that you son has Eosinophilic Esophagitis and is experiencing constipation. In general, constipation in children can be frustrating for parents. You should follow up with your primary care pediatrician
to discuss this issue, especially given his extensive allergies.
First, dietary modification can be tried. The most effective changes are increasing the amount of fluid and fiber your child takes in during the day. This is an easy step that is often suggested as a first step for most children with constipation. It is also important to establish a toilet routine such as having the child sit on the toilet twice a day for about 5 to 10 minutes after meals in a no stress setting. Adding prune juice to the diet is an excellent method to help soften stools. The amount of prune juice should be adjusted based on the age of the child. While the prune juice helps to soften stools, this is not an instantaneous solution and takes a few days to have a significant effect. Other juices that are high in sorbitol such as apple juice are also somewhat effective. Karo syrup can be tried as well.
Dietary modifications and the addition of prune juice or karo syrup are the most beneficial non-medication based treatments for constipation. Nevertheless, you should follow up with your son's primary doctor
to discuss his symptoms, possible underlying etiologies, and best strategies for treatment given his extensive allergies.