Cancelling an Easyjet flight for one person

My oldest son has applied for an apprenticeship and has been told that he might have to start at the end of the month if he gets it. However, I have booked a holiday with 4 return flights from Bristol to Portugal (going 16th August, back 27th August). Am I right in thinking that if my son doesn't go, I would not be able to reclaim back his airfare? Also, as I have booked 4 flights as a family, would Easyjet still allow the rest of us to go? My son says that if he can't go, we should be able to still go - do we have to tell anybody at the airport and if so, will I get a fine/charge/fee on the day? Thank you.