The short answer is that you should not, and involving the attorney in this creates problems that are significantly worse than the problem you are trying to solve.
Attorney-client mail is privileged, meaning it is supposed to be opened only by the inmate and is not subject to the same mailroom inspection as regular correspondence. Some people try to exploit that privilege to get items into a facility that would otherwise be rejected. Facilities are aware of this and take it seriously when they discover it. If explicit photos are found in what is supposed to be legal mail, the consequences fall on everyone involved. The attorney faces professional disciplinary action that could affect their license. Your inmate faces disciplinary consequences inside. And you potentially face your own legal exposure depending on the circumstances.
An attorney caught passing explicit photos of their client's girlfriend through privileged legal mail channels is not a story that ends well for anyone. It is also a significant breach of legal ethics that no attorney with a functioning law license should be willing to take on.
The practical alternative is already in front of you. Send photos that are suggestive without being explicitly nude. Lingerie, tasteful but clearly intimate images, the kind of thing that communicates what you want to communicate without crossing the line that gets mail rejected. Those photos can be sent through InmateAid or standard mail and will make it through the mailroom at most facilities.
He knows what is waiting for him when he gets home. A photo that arrives and stays in his possession is worth considerably more than one that gets confiscated or causes a problem.