I think if your son is the top 1-5 in the whole grade plus sport, music or maths' achievement. I agreed with Kenfu, it also depends how many classmate will apply this school too.
It also depends on which primary school your son is studying. If it is a band one primary school, the chance is high. If it is a normal primary school, the chance may not be high as St. Jo is very hard to get in.