A kid that young in a 5th grade class, I would not just let him figure it out. I'd want a relationship with that teacher ahead of time.
Absolutely make contact with a kid this young. My ds was 12 going into a Math class at the HS with kids 2 to 6 years older. I was significantly more hands off because he was older. I went to the parent information night and introduce myself. I let the teacher know I would not be hovering over him or his work. If she saw anything concerning to please contact me. He was older and I felt it was important for him to understand the expectations of the class would be the same for him as any other kid in the room. Socially, he made great friends and it is a non issue, but again the age group is very different. It was also an Honors class and the kids in there are more interested in doing well than my ds's age.
I would contact the teacher and try to get a feel on his/her attitude about your ds. Consider asking her for any expectations in EF area and work on any weaknesses with it over the summer. Ask for other suggestions the teacher may have for the transition to be as smooth as possible. It starts the relationship on a positive note of you asking for advice on how to best prepare your ds to be just like any other student in the class. Social issue is probably a wild card depending on the students and teachers response to it. The kids are old enough to look pretty bad picking on a 2nd grader and hopefully that's a deterrent for any nonsense from them.
Another thing my kids sometimes do better and feel less stresses when I over play the difficulty of something. When Ds went to take ACT I told him it is super hard and lots of people don't finish it in time. It lowered his anxiety when he had to guess on problems. He walked out feeling that was easy. Not a good idea for everyone, but it works with my kids. They come away from the conversation understanding it will be difficult and my parents don't expect perfection. It works great with my ds. My dd6, I recently noticed the more I tell her something will be difficult the more she response with "a rise to the challenge attitude." She started reading Lemony Snicket books last week because I told her they were probably to difficult
Maybe prep him emotionally over the summer with things that work to motivate him and reduce stress for him. If nothing else he knows you are expecting him to not be perfect and just be himself.
