On my aunts farm the postman used to just walk in and leave the post on the table if they weren't in the house, now they tend to lock the door and the one key is left in the outside loo for everyone to let themselves in as they need to.
My parents front door isn't locked (during the day when they are in) and you could just walk up and let yourself in. As they grow a little older I'm a little more worried about that one as they live on the outskirts of London, but still have that farm mentality when it comes to doors!
At my old house I used to leave the front door wide open if in the house in summer, and the postman used to shout hello as he chucked the post through the door, but it was in a quiet cul-de-sac.
Bad luck to your son though, is he insured? Though that doesn't get over the feeling over being broken into (yes the old house was broken into but when we were out and hadn't left lights on, which I always do now).