You can put QC and another school as the two choices at Discretionary Stage of SSPA and keep LSC as your first choice at Central Allocation. This can ensure you to get the LSC place if you fail to get an offer at Discretionary Stage.
You can just put QC as the only choice at Discretionary Stage as once you get an offer at Discretionary Stage, you will lose the LSC feeder school place at Central Allocation.
QC tends to select top boys from different schools. Hardly can you see a top primary school have more than 3 students allocated to QC (except for affiliated schools or schools in Wanchai via Central allocation).
Your son is probably the only LSPS boy applying for QC. He will have a good chance if his EDB ranked order is high and he behaves decently during the interview.