NOW! that is that is the way it should work. If you think there is hanky panky call it. it's a $100.00 down. Keep in mind there is a chance the seller could come after them legally. But as for getting out of the contract. I don't think and agent can make the call for to me it sounds like legal advice. An Attorney is the only one that can make that call. Take the contract, inspection report and every thing you have to an attorney. Mind you I am pretty sure the house being located near the interstate will not be a reason to not close but I think the damp corner or the low spot in the kitchen floor is. If anything else make the seller fix the issues they will not and you won't close. But this all needs to be done by an attorney. If you need one, let me know.
I am curious as to why he didn't use the home inspection time period to figure out that he didn't want the house - he could have used the inspection as a reason to back out? But, too late now.
He could still back out, but would probably lose his $100 and risk getting sued for specific performance (to complete the deal) or actual damages/court costs. If it was me, I'd offer the seller a few hundred dollars for the inconvenience and ask him to get out of the deal. He may just let him off easy if he feels compensated. That would probably be cheaper than trying to resell it after closing or going to court. And there's no way around it - your son was the one who screwed up and is trying to back out.
By the way, home inspectors don't work for brokers. You didn't 'have" to use the recommended inspector. They are independent contractors and the BUYER is the client. The things you have listed would probably not have been considered significant enough defects to get you out of the deal anyhow - unless the "low spots" were indicative of larger problems. If the home inspector missed real defects, you could probably go after him for damages - but I'm not a lawyer, so this is just anecdotal commentary. Some inspectors put disclaimers in absolving them of any responsibility of missing defects. The good ones have Errors & Ommissions insurance and of course, cost more.
Your son should have had a buyer's agent helping him. As a buyer rep, I would have pointed those things out before he even wrote the offer!
Either way, get a lawyer or try to negotiate getting out of the deal with the seller. It IS possible, but the agent won't tell you that, because she doesn't represent you. It's her job to represent the seller's best interests.