Have a look of RC HKDSE results and compare it with SFA's for a better comparsion. RC HKDSE results were very good to have 3322 with 78.3% in 2013. For your information, In 2012, Wah Yan HK College had 74% met the university entrance requirements, ie 3322.
In my opinion, it is better to study at RC and then see if you can switch to other top school (if he is doing very good at RC). If your son go for SFA and his academic is just so so ..... at SFA ....... your son will have no last line of defence !