It is so wonderful to hear about the work you have been doing with Kristoffer! How amazing that you have helped him expand his language!
Some ideas of things to have in the work room would be paper and markers, different books and magazines, things to do self help skills with (e.g. items to shave with, hairbrush, etc). You could experiemt with one of those foot spas if you have one. Balls/badminton/jump rope,any sports games. Weights for working out, musical instruments, anything to dress up in (e.g. wigs, sunglasses, hats).
We would definitely recommend keeping a couple of shirts in the work room as it sounds like rolling them is his main ism. We do not want to limit his ability to ism because it's his way to gain control in a world that it unpredictable and to soothe himself. Building a loving realtionship with him by joining when he wants to roll the sleeves is the thing to do when he is not available to interact with you (e.g. not looking, paying attention to you, responding to you, etc).
I would start with having interactions that don't involve school work, just to get some kind of interactive attention span going with you when he us ready, One idea is to bring in some basic white shirts (you could get some cheap ones from the thrift store). Then when he gives you the green light (is looking, talking or paying attention to you in some way), you can draw a design on the shirt with the markers in a fun way. If he responds, invite him to decorate a shirt and then you could both wear them once you have finished. If he likes it, you could advance to doing Algebra sums on one of them.
I hope this helps! have fun!
Son-Rise Program Teacher