Our recommendation is to first obtain all of the documentation your son may have relative to this and carefully review it. This may shed some light on your options from a non-professional position.
Consider putting it in writing that you are now considering taking your cause to the legal level. There are a couple of avenues. You first option may be to be in touch with an attorney for their advice and it may be as simple as them drafting a letter.
Your second option is to explore what in most states, is referred to as "small claims court" which allows a party that feels they have a legal claim, to present it before a judge for their decision. Additional information may be founf on line.
It sounds like you should actually give the 17 references to to them so that they might secure a replacement lease. Just telling them that you know someone that might rent it is not the same as securing another lease with deposits paid.
I would try to get them commit to their original promise of returning the deposit if they get another lease. Get this in writing, and them give them the 17 names.
Other than that, you may need to contact an attorney if you feel it would be worth your time, effort, and costs involved.
Best of Luck,