In general, unless the sexual act was with a child under 14 or was violent, the requirement to register ends 10 years after the completion of the deferred adjudication that your husband received as an adult. (The law varies based on the age of the complainant, the level of violence-if any, and if the defendant is a minor. In Texas, a defendant is an adult starting at age 17 for purposes of criminal cases.)
It is sad that our laws are so strict because as you commented, 3 months difference in either of their ages could have prevented the charge assuming that the encounter was consensual. (In Texas, it is not a violation of the age of the c/w is within 3 years - to the day - of the defendant - as long as the act is consensual.)
If your husband meets the 10 year time period (that is 10 years have passed since he completed probation), then he should talk with a lawyer about registration.
Congratulations are in order to your husband for successful completion of such a difficult probation. The laws & enforcement make it almost impossible to successfully complete a probation because of the therapy requirements to admit a crime and polygraph requirements.
I do sympathize & understand about the effect on you and your children. The laws affect more than just the defendant, as you have sadly had to deal with.
As long as you continue to think that there is nothing objectionable about an 18 year old man having sex with a 15 year old child you are going to have problems. You need to speak to a TX attorney about the possibility of being relieved of registration.
There are statutory provisions pertaining to removal from the sex offender registry. Whether they will apply to your husband's case will need to be reviewed under the statute in effect in your State of residence to determine whether your husband is eligible to petition for this relief. The petition has to be filed (the relief is not automatic); and, given the public interest in these types of registrations, the petition will need to meet the applicable State's provisions for relief to have any chance of success.
That being said, I suggest you check out the lawyers who practice criminal law in Texas who list here on Avvo.com, check out how they answer questions like these, pick out a few who have experience with these types of cases and call the ones that seem like a good fit for a consultation. Most will give you a phone consult (which is probably a good thing given the size of Texas).
In the event that you do not find a suitable lawyer through Avvo, try the listings at Findlaw.com and Lawyers.com which are both very large marketing sites for lawyers (even if they are both paid directories, unlike Avvo.com). Good luck.
This answer does not create an attorney-client relationship or constitute legal advice. Instead, given the nature of this website, it is provided solely for informational purposes, for you to use as a starting point when speaking directly with a lawyer in your State. Do not assume that the legal theories I mention that pertain to NJ will apply in your State. The laws of each State; and, the facts of each case are different, and it is therefore critical for you to consult with a lawyer admitted to practice law in your State before making any decisions on how to handle or dispose of your case.