whatsapp has disappeared and states restrctions enabled

We’ve set up a phone for our son and have downloaded WhatsApp for him and it worked fine. Last night his phone updated to ios15.5 and WhatsApp has disappeared from his home screen. We have tried to open it from the App Store but get a message stating Restrictions are Enabled and to use the app we have to turn restrictions off. It is an allowed app so don’t understand what has happened or how to fix it.