Could he have been photographed alone because he maybe has no friends so no one to stand with?
Whenever we had photos at school we always stood with our friends, I've still got all my photos from school events an am with a few kids (many that I still keep in touch with today)
He's been in an out school since he started, it would be hard for him to keep friends since if he's not there everyday kids will just leave him, most of these kids have probably known one another from daycare or mommy an me clubs as well, it would already have been hard for him to make friends but it will be even harder to keep them when he's constantly getting pulled out