Perhaps you need to look at both schools SEN policy and their provision and history. You also have the option to withdraw from the mass and you need to read up on your rights. No good using this as a reason not to go if infact you don't need to attend.
I would approach the arguement as if you were preparing a school appeal. What does the school you want offer that the others don't?
Look at the setting of the schools, areas of quiet where some autistic children perfer to learn. Look at the accoustics of the school, how lunch is served and again the noise and how it is organised. The space provided, access to outdoor or forest activities. Look at their behaviour policies and if you can access the last parent questionaire results on previous Ofsted reports.
Look at the pastoral care. Check out the section 48 report too.
What support do they offer. Are there clubs suitable for you son? You need to show the school you want is more molded to cater for your son. They have to consider best value for money and sometimes the cheapest is not always the better option in the long run. Look to see what mainstream school was unable to provide and work from there. Is there already a transport system in place close to you? Is there another pupil close to you where transport costs can be reduced?
Look at his social needs and ability to make friends. Is there anything to help him mix and make friends. Are there clubs that he is confident to join and does he have skills he can bring and share with others? Look at how independence and individuality is nurtured. Is the school bright, relaxing, stifling, dreary, inspiring, secure? Did the person showing you around instil you with confidence? able/unable to answer your questions? Were they interested in your son? Were the lessons chaotic/ interesting? Were the students involved, out of control? You have a right to put over your
impression of the school because often gut feeling are correct. Build on your overall sense of the place and why it would/would not suit your son.
The Cathloic symbolry scaring him would be valid and another barrier to get over along with the usual worries of a new school. His thoughts/ impressions and views also carry weight and should be included.
I would consider the school as a whole and not just the class/educational bits. Have you spoken to other parents of children at the schools?
Is there a bus route he can access when he is older to help his independence? Look at the trips previously arranged and the opportunities for residential trips. These are all part and parcel of the educational experience. Think outside the box. All the best.