For us, it was the outside of school stuff that helped the most. MS in FCPS (at least theoner my DCs attended), are run more like prisons. Little time to socialiation and little time to get into trouble. The buses are the most vulnerable time for a kid low on the totem pole. I ended up drving and picking up DC most of the time. Eventually, he was able to take the bus. The exception to this is band, orchestra or chorus since they have trips. Although DC is in orchestra, I would reccomden a band instrument as marching band is excellent for kids who need more socialization. Orchestra doesn't have the camaderie that band has.
Outside of school we did as much socialization stuff as possible. DC continued with scouts, church youth group and attended a weekly social skills group.
He is now a Junior in HS and has found his social niche in our congregation's youth group. I have seen others find their niche in scouts (boy and girl) and others in band.
Good luck with your DC!