I walked in from the USA just as easily, despite the fact that the value of my 'gifts' was probably a little over the allowance.
Airport security, customs, immigration etc. know who they're looking for. They have names and faces, they know the travelling patterns and behaviours of the people - or the type of people - who are likely to be smuggling big stuff into the country knowingly.
If your son had been up to anything illegal, chances are that it would have been picked up at the other end, and he would have been monitored throughout the journey as the authorities gathered evidence (remember, whilst it may be a crime to take stuff into countries, it may not be illegal to take it out).