I didn't think that any head/school could GUARANTEE a place at their school. Not trying to worry you, I'm sure it'll be ok if he passes.
But as I understand it every school has an oversubscription criteria. Some are distance from home - others such as Rochester Girls School take in rank order of the 11+ test such as you were mentioning earlier Lorraine. You need to check your admissions booklet to see what Rochester Maths school oversubscription criteria is.
Also, this year no school will actually be allowed to know WHAT order you put on your CAF form, the LEA will just try to place your child at the highest possible school on your list that offers a place.
Hope this helpful.