I think you may have a problem finding a school with a particular focus on art in Hackney- and unfortunately, it is very difficult to gain a place in an out-of-borough school unless there are specific reasons such as proximity, behaviour or SEN issues.
Having said that, the beauty of living in Hackney/Tower Hamlets area (as I do too!) is the amount of artists and galleries and collaborations with local schools. As an art teacher, one question I would be asking of an art department is 'Which outside agencies/galleries/artists have you worked with?', as this shows a real commitment to creativity. I'm not sure if your son goes to any workshops or classes in the holidays, but the Whitechapel gallery has some excellent resources and can point you in the direction of workshops etc. for children his age. Ditto the Museum of Childhood in Bethnal Green...
OK, as far as Hackney schools go, Stoke Newington school has the best reputation for art, although it's been a few years since I did supply there; I thought it was a pretty good school, and I hear it's improved a lot.
Chestnut Grove school in Balham is amazing, and has specialist arts status (although this is not necessarily indicative of a good art department)... but I think a journey from Hackney would be way too far..
Hampstead School (not to be confused with South Hampstead high school which is private) is excellent and has a really vibrant art department.
Fortismere in Muswell hill has an excellent art department also, but you have to live on the doorstep to get a place
On the whole, I think you will be able to suss out a strong art department very quickly- look out for examples of 3d work, conceptual work, visits to galleries, contemporary work, varied work (rather than a production line) and NO COPYING FROM PHOTOS (sorry, my personal pet hate- and a sign of lazy art teaching!), and ask if they stage exhibitions in or out of school.
Unfortunately though, the chances of getting in to anything but the local state school are very slim; unless you move to the catchment area of your preferred school or gain a scholarship to a private school (I have no experience of private schools) your options are limited.
Having said that, I think Hackney has a great deal to offer, has loads of funding being poured into schools, a great vibrant arts scene and a really cosmopolitan feel, which could be a more positive influence on your son's arts education than a school that looks great on paper, but is actually no more than a GCSE boot-camp. (Don't trust the OFSTED report, especially if you are interested in art; most inspectors do not have a clue how to assess an art department. Visit and make up your own mind!)
Hope that's of some help...