but he would practise the Christian faith at a Christian school which would be different to the faith he currently practises
Not necessarily - if the DC got in, the OP could remove him from the worship / RE aspects. Although that would seem to be a bit odd, having made such a fuss about him not being allocated a place to start with.
The Admissions Code make only two real stipulations with regard to the setting of oversubscription criteria, basically that looked after / previously looked after children should be the highest criterion and that any other criteria listed must be clearly understandable, and reasonable.
For a faith school, giving priority to those of the faith concerned is allowed. Giving priority to those of the faith worshipping at the church in the parish where the school is situated is allowed and reasonable in that it prioritises local DC (of the faith) etc. If the school does not fill all its places before it runs out of 'own faith' applicants, then it must admit those who don't fit any of those criteria (members of other religions, those who live miles away but want this school / any faith schools/ just a school place, please). But it must apply its oversubscription criteria in order, and stop offering places when PAN for that intake is reached, whether in the middle of criterion 3 or with the second-last 'any other applicant' (unless the last one in is from a multiple birth, when admitting the other DC over PAN is allowed without the school having to take the 'relevant measures' that just randomly deciding to take a few more would do).
@welcome13 what are the actual oversubscription criteria for this school? Is there actually a criterion along the lines of 'member of another religion, with regular worship evidenced by a statement from the minister (or whatever) of the place of worship', or are members of other faiths, however devout, just included under ' all other applicants'?