What can I do about the excessive length of time in the SHU?

Unfortunately there is nothing anyone on the outside can do to intervene on prison matters. Discipline and punishment is meted out by the rules of the institution and all the inmates know them the minute they are first brought in. If there is a suspicion, the system has it's way of doing things and it will have to run it's course.

He will have a disciplinary hearing where there are no lawyers speaking for him, he will have to convince the hearing officer of his innocence. Some inmates do a year or more in the hole and come out of it fine. There is nothing inhumane about the hole other than you are being punished for breaking the rules in prison - the consequence is that they are taken out of the general population and placed in isolation, their commissary is limited, their visitation is limited and their phone time is limited - but they have a bed and three square meals a day. The best way to help him is to send him lots of mail with pictures of you and reading materials; he is probably going to be there for longer.