Don't beat yourself up about this, stuff happens and double basses, being big and poorly balanced, are prone to this kind of accident. Very likely your son will have other accidents with this or other double basses in the future. It's like a new car, the first dent really hurts. After that is over with, things get easier to cope with.
From friends who have experienced similar breaks as you have described with this price range of a double bass, I suspect your luthier will be able to find a way to fix it, hopefully at what might be considered "student prices". It may not look new in the repaired location, but I would guess once it is repaired it would be solid and not rattle. Your luthier will probably suggest that he also take the time to double check other areas that may have been affected by the fall, which is a very good idea.
Keep in mind, if the price is too high, you can check around for another quote at least to verify pricing. Reasonable prices will depend on how bad the damage is and how long it will take to repair. There is plenty of lutherie related wisdom to be found here among our active luthier community, too, but without pics and without knowing exactly what broke, it's hard to say much and you're pretty much dependent on your local luthier.
Once fixed, it might be worth it to consider ways to store this double bass so it isn't quite so prone to falling. There are lots of options for stands, bags and cases.