Don't feel bad, there's a LOT of people that have made the same mistake.
But 'metal' is just about as generalized as a word as you can get!
Because 4041 chrome-moly steel is a metal, but pewter (90% tin and the rest is copper, antimony, bismuth and lead) is also a metal. So are zinc alloys, magnesium alloys, Chinese alloys (or lead alloys as I call them).
The problem is, most airsoft guns are made from metals that by all means, should NEVER be used with any kind of force trauma.
For example, magnesium alloys, which are a favorite of some manufacturers, are really big in the computer industry.... for computer parts.... that sit on a shelf for 10 years undergoing almost no stress at all....
Airsoft guns are by no means real guns, you cant stand on them, cut down trees with them, beat people with them, break windows with them, or use them to break your fall. Infact it's generally inadvisable to do all those things with real guns anyway.... lol
He's just going to have to learn to take care of his gun better!
And having learned this only a few years ago myself, you should make your son pay for the repairs, because kids typically won't care about expensive things unless they paid for it themselves! Learned that one with my first car lol