I tried looking up whether there is any minimum age for boarding schools, didn't succeed, but found this.
In order to register an independent school, details of, amongst other matters, the age range of pupils have to be filed. If there are to be changes, the school has to "apply in writing or by email to DfE or the Welsh government for approval of any significant change to the school or its running. This includes changes to any of the information which was required when applying for registration." I'm guessing that the DfE would have a lot of questions to ask about why a boarding school wants to change its range to include a 5 year old; a statement that his stepfather says his mother isn't coping in the later stages of pregnancy would hardly be sufficient. They would also want to know how they would propose to teach and look him given that they would have no other pupils of that age.
Why on earth would any school want to go through all that hassle, no matter how much they loved their former pupil?