I do believe that at age 10/11 it's still ultimately the parent's responsibility but if you don't have any information to go against your DS' instincts then I would go with his rather than yours.
Firstly, he will be the one spending 7 years at the school and if he feels he 'fits' it will make life a lot easier. Also, there will be tricky times and if the choice is his rather than yours he is more likely to deal with them and has less grounds to blame you!
As the mother of one who is at SHSB and loving it (but who did dither over whether to choose his elder brother's old school or WHSB) and one who started off there but rather came adrift with the school, but moved to SHSG for sixth form and was happy there, I would agree that although the choice is ultimately your responsibility, if your DS feels strongly that Westcliff would be a better 'fit' for him then go for it. Have a look at the other options when he gets to year 11, perhaps. Dynamics do change and although most students choose to stay where they are for A levels, there is quite a lot of movement between the grammar schools themselves and between the grammars and the local comprehensives.