Sorry, could you just clarify whether your son needs to pass the 11+ to get into your first choice school? As he has a statement, are you sure you need to be living nearby? Surely if the statement names your 1st choice school as the best provision for him, you would not need to move? If the school with the ASD unit does not mention SEN in the admissions policy that sounds like an odd omission - presumably they do not have many applicants for that unit who do not have a statement?
Perhaps I am misunderstanding but in my experience if a school is named on a statement then that school is obliged to take a child. When is your statement due for review? Surely there will be a transfer review of this and the EP can make a recommendation on the most suitable provision for DC?
Sorry if I am being dense.