My son signed a lease but the lanlord didn't. He will not be moving in and informed them of this. Their response was that he had to find someone to take over the lease and pay an additional $175 (they will keep the $100 security deposit anyway) or he can just pay the 12 months rent. Please advise.

How do you know the landlord did not sign it? Have you seen the copy they have on file.

Personally, if that's all they want for him to get out of it, he's getting off cheap. They can hold him liable for ALL rent due until they can reasonibly find a replacement tenent which can be a few months.

But it does help to know what city state and country this is in as laws do vary.

I see him not moving in, the landlord sues him, it goes to Court and then the unsigned document (which is a contract) comes into play.

Of course, the fact that your son doesn't have a signed contract is meaningless. The landlord undoubtedly has the original signed by your son and now will sign his name, if he has not already done so. It will be your son's word against that of the landlord.