Hi,
One thing that helped my daughter last year when she moved into 3rd grade was for the teacher to pick one or two other students that were nice kids and good students to kind of buddy up with her, at least for the first several weeks. Could you ask the 1st grade teacher to do this for tomorrow or Wednesday? My daughter didn't know these kids before the transition, but it was nice for her to know she always had somebody to buddy up with at recess, lunch, small group time until she was able to make some friends.
The other suggestion I would make is for you to request the school supply list for his new class. We didn't do that right away (just didn't think about it) and, while it wasn't a big deal, it would have been one less thing for her to worry about and one more way to make her "blend in" more quickly.
How did it go for your DS today?