First, what is your boyfriend doing texting with someone he doesn't know?! Even if they said they were related to you, don't you find that discussion inappropriate on a moral level. About the criminal case, he has not stated anything sexual or otherwise for which he can be prosecuted. They did not agree to meet for an illicit purpose, he did not threaten her with harm, he did not even use sexual language.
Being in a small town, law enforcement may believe they can "do something" for the incident. As some say, he may be able to beat the rap (criminal case) even if he can't beat the ride (charges, bond, etc.)
I would talk with a local lawyer who may be able to be on call in case the police want to talk to your bf. I would be cooperative with the attorney & his advice, and I would be nice in my dealings with law enforcement. I do not believe a crime has been committed, but there is no sense in riling everyone up. Perhaps an apology to your brother (after consulting with the lawyer) would be a good solution.
Under the Texas Penal Code 33.021, a person who is 17 years of age or older commits an offense if, with the intent to arouse or gratify the sexual desire of any person, the person, by text message or other electronic message service or system, intentionally communicates in a sexually explicit manner with a minor. If the minor is under 14, then the crime could be a second degree felony punishable by 2 to 20 years in prison or up to 10 years of probation and a fine of up to $10,000.
The State would have to prove that the communication was intended to arouse or satisfy the sexual desires of your husband and the communication(s) was in a sexually explicit manner. The second part would seem to be the hardest one for the State to convince a jury of. Talking about tickling and peeing your pants is not something that most people would consider sexual in nature, but it depends on the context of the total communication. That being said, Ms. Henley is right when she says that your husband may be innocent of this charge but still subject to investigation and possible arrest. He and you should avoid posting any more information about the case online.
Your husband should speak with a local criminal defense attorney, especially if the police try to question you.
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.