You bought a movie DVD that you made unusable. You now want to watch the movie. Hmm. It seems that you're in a bind.
Why should it be lawful for you to reproduce someone else's copy of the movie?
Because DVD's are easy to scratch? Even if true, that was part of the deal when you bought the DVD.
Because you already bought the DVD once? Nope. If you buy a light bulb but drop it before you get home do you have the right to go back to the store and take another one?
Because it would be so easy to freely download the movie and the DVD maker wouldn't be harmed? Ah, there's the rub. The fact that you once bought the DVD and your son scratched it and they're easy to scratch and the law harms the innocent -- all of that is irrelevant, peripheral dressing. The meat of the matter is that you want to make a copy of a movie without paying for it.
You can't, at least not lawfully.
Nor should you be able to. At the moment, it's part of the business model of the movie industry to make money selling DVDs. While that may change [perhaps to a streaming subscription model], at the moment the wholesale copying of movies w/o compensation to the movie industry would seriously harm their business. So, no, there isn't any way you can fight for the "right" to reproduce a movie because you destroyed your DVD copy.
In your case, the law does not expressly help you, given the common use of DRM on DVD's and the fact we are talking about movies here. However, there is a right for the "little guy" when it comes to music and software. Backing up is technically legal under 17 U.S.C. 117 for software, and 17 U.S.C. 1008 for phonorecords and sound recordings, provided you are not bypassing any copy protection. You may also want to see the RIAA v. Diamond case. They key is to be proactive before the DVD is destroyed, not after.
I hope this helps.
Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.
Record labels and movie producers have deals with their distributors that allow them to withhold "reserves" from sold products to account for all the people that return the product for one reason or another. Some retailers have strict return policies, some are no questions asked. It's possible, but unlikely given that you've had this DVD for years, that you could return it for a refund or new copy to replace the one you had. Returns are supposed to be for defective products, but again, some retailers have very liberal return policies and if they can get reimbursed from their distributor, they'll give you your money back or a new copy.
if not, can you fight to make it possible to download a copy for free? You could write your congresspeople and advocate for their to be some kind of "lifetime" copyright right, or maybe no copyright at all, but that second idea contradicts the U.S. Constitution. Those that think that there should be a lifetime copyright right, and once you've paid for a copyrighted product, you own it for life, including the right to make multiple copies and put it on multiple machines and get replacements if/when you lose or destroy the one you bought, are really asking for a lifetime guarantee on a product that's not made to last a lifetime, and is priced accordingly. Are willing to pay more for more durable DVDs?
The way copyright law is written now, the owner has the exclusive right to reproduce the work, and buyers only have the right to enjoy the one they bought, and if it wears out or gets ruined by the sun, the cat, or a 4 year old boy, that's life.
Every copyrighted product that's stolen (not paying for something and taking it any is theft, and there's not really anything innocent about it) shortchanges the owner, who in turn can't pay royalties to the talent, which ends up, in the long run, causing people to lose their jobs. Individually piracy amounts to pennies per copy, but multiply that times 1000 or 1000 or 100,000 and it's real money.
Can you download a "free" copy? Yes, and it's disturbingly easy. Should you? No, and you should use the situation as a teachable moment so your son understands the difference between right and wrong.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.