Your expert asked me to take over your question, which I am happy to do. That said, I am sorry to hear about your son's arrest, and I suspect that you do not need me to tell you that he is facing very serious charges. As sex crimes go, this is the most serious of all of Nebraka's sex offenses, and the maximum penalty for it would be 50 years in jail, to say nothing of lifetime registration as a sex offender. Here is the law with which your son is charged.
Please note that although you should be very, very concerned about this case, there is a vast difference between what police need to make an arrest and the state to file charges -- which is only a reasonable belief that illegal activity may have occurred -- and proof beyond a reasonable doubt, which is the burden the state must meet in order to get a conviction. So don't lose heart.
The age of consent in Nebraska is 17. That means that according to the state of Nebraska someone under that age does not have enough experience or knowledge to consent to sex. That makes a consent defense impossible, (except to the extent that your son didn't have to use force). No 14 year old in Nebraska can have consensual sex. Any mother of a teenage girl, can probably tell you that they are far more knowledgeable than states want to give them credit for. But unfortunately in terms of consent, it does not matter.
There is also another difficulty. Some states -- not many any more -- recognize a reasonable mistake as to age as a complete defense to an act of this sort. Nebraska does not. That doesn't mean that the fact that she looked 18 and may or may not have lied about her age won't be relevant. It does mean that it won't be enough to completely exonorate him of this offense. A better defense is likely to be that they hadn't had sex at all, as that is what the girl is saying. That, of course, is what's most in your son's favor.
You will need to get him an experienced lawyer. This is a type of case in which the quality of the attorney can make a difference. If you cannot afford one for him he can plead not guilty and ask for a public defender. They are plenty experienced as well and generally have tried many cases, but as you can't pick the defender he'd be assigned and they can be difficult to reach because they are always in court, you will probably feel more comfortable with someone more accessible.
You can contact the State Bar Association's Lawyer Referral Service for a reference. They guarantee you an active member of the bar in good professional standing and their referral fee of $50 or less will include a half hour appointment with the lawyer. There are also very reputable referral agencies such as martindale.com an avvo.com where many lawyers list their credentials in order to solicit customers. Many legal profiles show ratings by actual clients, so that could be of assistance to you.