Hello,
Generally you can apply to the school of your choice but whether or not your son is accepted will depend on where you live (how close to the school you are) and how many children are applying to that school. I think Manchester has a grammar school system which means there might be certain schools which select pupils based on their academic ability.

Start by contacting the local council and they can advise you:

http://www.manchester.gov.uk/info/100005/education_and_schools