I am pleased your son's gifts have been recognised. It is not unusual for children with aspergers to have talents in very specific areas. (I like to think of them as compensation for the things they find difficult.)
I have to say my experience of secondary schools is quite broad and so is their response to talented children. There is understandably an emphasis on helping children with specific difficulties which thwart their progress, but a good school will also acknowledge and support the gifted.
Firtsly, you need to confirm with his primary school that they are going to clearly communicate the detail of his gifts and emphasise their findings to his next school. It may also be an idea to ask them if they know how good that school is at stretching more able children.
Secondly, if the first parents' evening is not for some time in the secondary school, request a meeting to discuss your son's special needs - regarding his talents and his aspergers syndrome. The key to a successful relationship with his secondary school is good, open lines of communication.
(To be honest, I am surprised you have only just found out about your son's talents. Were these not evident last year?)
As far as the register is concerned, it is a formailsation of his talents. It should mean that the school look specifically at his learning needs and create an individual approach/plan which considers his gifts. Ultimately, they should make sure they stretch and challenge your son; Making the most of his skills, whilst ensuring work in reading and spelling is geared to keeping him interested and inspired.
Sorry about the rambling.. Hope this helps!