I don't have DC, but this was me as a child - I was miserable. Bullied by most of the class all the way through primary, not low level either, as well as at a school that was completely unable or unwilling to differentiate work for the above-average. Sometimes I wish my parents had moved me. I understand why they didn't, though, and am not sure it'd have made much difference.
For secondary school I chose to go to a school on the other side of town without any of my primary classmates. I wanted to go because I loved the school but leaving all the nasty bastards behind was a definite bonus. It was the best thing I ever did, the first year at secondary was hard because I didn't cope well with change and transition as a child (still don't, tbh) and there was some low level bullying. However this was a really good school who came down on it like a ton of bricks, it stopped, and I had the most brilliant secondary experience after that.
I would be very, very careful in selecting a secondary. You want somewhere he likes the look of (if he's enthused about going it'll hopefully show in his confidence when he gets there), that's also good with pastoral care, as he's had such a hard time. Finally it needs to suit him academically because at least part of the reason I had such a good secondary experience was that the place suited me down to the ground, they were great with kids who loved to learn and had a strong MFL/liberal arts focus. If I'd been more sciency I'd probably have picked the other secondary a few streets over who had a strong STEM reputation but the same sort of 'feel' to it otherwise.
Do you have a good selection of secondaries? Will you be able to choose between, say, school where all his current classmates will go vs. one that allows him a fresh start?