Adverse possession takes a very long time, if it is allowed at all under MS law...typically 20, 25 even 35 years. Your situation is not really adverse possession, but a case of minor trespass.
I advise you to speak to an attorney in MS that can help you with your options. Sometimes there is a way to go into civil court and address this issue, sometimes it has to go to municipal court as an offense (a minor criminal matter). Also, in many communities there are dispute resolution boards that are intended to hear neighbor disputes like this without starting out in court.
I am not licensed to practice law in your state, so my answer should be considered general guidance based on broad legal principles, not legal advice in your case. You should consult with an attorney licensed to practice in your state who specializes in the relevant area(s) of law for a definitive answer to your question.
My answer is not intended to create an attorney-client relationship and is not confidential. Pursuant to Circular 230 of the Department of Treasury: (1) no written statement to be provided by me relating to any Federal tax transaction or matter is intended to be used, and no such statement can be used, for the purpose of avoiding penalties that may be imposed on the taxpayer, and (2) such written statement may not be used by any person to support the promotion or marketing of or to recommend any Federal tax transaction or matter.
Adverse possession takes ten years in Miss. and there are a number of additional requirements, such as: your neighbor would have to hold the land against you hostilely (which he is doing by not honoring your request), against your will and be open about it (such as declaring it is his land) and he must claim he is lawfully entitled to it. If his fence is a little over on your land then you might have a problem after it has been there ten or more years, but the space where the truck and junk are can be resolved by calling your local law enforcement office.
Almost all municipalities and counties have laws against junk in a yard and you can file a complaint with your constable, local police or sheriff.