JOEL - There are no scriptures that state that a person who does not pay tithing is going to Hell. Being damned is not really the same thing as going to the place we call Hell. Actually the word "damnation" is a term derived from the Latin damnum, meaning "injury" and "loss," and connotes deprivation of what should have been possessed. In LDS doctrine, to be damned means to be stopped, blocked, or limited in one's progress. People are damned whenever they are prevented from reaching their full potential as children of God. Damnation is falling short of what one might have enjoyed if one had received and been faithful to the whole law of the gospel. In this sense, all who do not achieve the highest degree of the Celestial Kingdom are damned, even though they are saved in some degree of glory. They are damned in the sense that they will not enjoy an eternal increase or the continuation of the family unit in eternity (D&C 132:4, 19).
Therefore a person can be damned by not paying their tithing, but they will not necessarily end up in Hell. They might even still end up in a lower level of the Celestial Kingdom. But the eternal marriage covenant they made in the temple will be invalidated and they will not be with their family forever nor be able to become as God.
So tell your husband to relax. Being damned is not really as harsh as it sounds. It only means that he will not be able to get as far in heaven as he could have gotten. God is a loving, merciful, and forgiving Father. That is why He calls it the Plan of Salvation and not the Plan of Damnation. We will all end up in that level of heaven that we will be comfortable and happy in. But remind him that he may be there alone without you if he doesn't pay his tithing. That would be a bad thing. Right? :-)

The main reason why we have the law of tithing is simply because God has commanded it. By revelation to the Prophet Joseph Smith, the Lord stated that members should pay "one-tenth of all their interest [increase] annually; and this shall be a standing law unto them forever" (D&C 119:4).
James E. Talmage said, "The prime … purpose behind the establishment of the law of the tithe is the development of the soul of the tithe-payer," (AoF, Talmage). It helps us build faith and a strong character.
The reason why we should live the law of tithing is because we love God and want to help Him spread His kingdom over all the earth.
There are penalties if we don't pay it, but there are blessings when we do. President Joseph F. Smith taught that the disobedient "have cut themselves off from the blessings of Zion," but added that the Lord will fulfill his rich promises to the faithful tithe payers of the Church (Gospel Doctrine, pp. 225-27). And the Lord will open the "windows of heaven, and pour you out a blessing, that there shall not be room enough to receive it (Mal. 3:8-10).
The payment of tithing is one of the evidences that a person is worthy of receiving the ordinances of the temple. If a person has enough faith to pay their tithing, they have enough faith to live up to the covenants and promises they will make in the temple.
Only the person who has paid their tithing and has experienced the blessings of doing so can understand how it all works. It is a doctrine that requires faith to know that God will provide for us if we will provide for His Church. In time a person who has faithfully paid his tithing and has experienced the blessings of doing so, no longer needs to do it on faith, for they now have a perfect knowledge and understanding of the principle and they continue to do it without even thinking about it.