Ask if there are any addresses he needs for other programs -- I wouldn't give him other people's addresses without their permission, but you could give him addresses to prison support programs.
I told him about day to day stuff - what my husband and I did over the weekend. If I was in his shoes, and even if the first letter from a friend was completely tone-deaf or caused me to further grieve my lost freedom, I'd probably treasure it.
There are also vendors that pack boxes with products allowed in prison meaning you can order a box with categories of items like personal care items, shelf-stable foods, etc, and they'll coordinate getting the products to your friend. And that will mean a lot. One of the biggest worries that people who decide on inmate release have is the environment the prisoner will be released into.
So, just be chatty. If you know this friend well, you should already have handful of things that you enjoy discussing. Persuade him to become spiritual and work on any personal issues he may have had leading up to his arrest.
You don't need to say, "You're in jail! It doesn't have to be deep, meaningful, heart-to-heart stuff. Suggest that he learn a new trade or read books from the prison library. For now, I'm sure he'll appreciate warm chatter about your daily doings. It's so good of you to stick by your friend!
Just really good jokes.