You might want to base your decision on what high school you want him to go to in 9th grade. He would then move on to HS with his new friends. I'm biased, but I much prefer South Walton HS to Freeport HS. I think they have more AP classes. There are quite a few people that live just north of the bridge that have their kids go to the schools in south Walton, so he wouldn't be the only one.
I don't want to make your decision any harder, but there is also a middle school in Seaside that he is eligible to attend. The entrance is via a lottery system, who's deadline has passed, but you never know which kids are going to move over the summer and a spot might become available. You can get on the waiting list now. The kids from Seaside generally move on to South Walton HS. The Seaside Neighborhood School is a charter public school and has a total of about 110 students for all 3 grades. My kids both went/are going there and love it. Another option for you!