My son Cole just started first grade this year as well. Cole turned 7 in July, so he may be a year older than Sam. We waited a year after he was eligible before sending him to Kindergarten. We're planning to talk to his class, but are waiting a few weeks for the kids to settle in. I spoke to his teacher before school started, and I think we're going to do a talk about differences in general, probably reading a book followed by a discussion. So it won't be a complete focus on Cole and DMD. He'll also be in the room.
In Kindergarten we didn't do anything, because I was worried about pointing out his differences if they weren't obvious. I volunteer at the school quite a bit and watching Cole it seemed he kept up really well with the other kids. No one commented or asked any questions to the teacher all year. But, toward the end of the year I was in the lunchroom and some kids started asking me why Cole was the slowest runner in the class. So it was obvious they did notice, even if they hadn't asked the teacher or commented in class. That made me feel we definitely needed to do something this year. I'm looking forward to hearing others suggestions.