Myself, I would not force the writing during the summer if he hates to write. If social skills are priority 1, I'd put my focus there, with lots of practice opportunity, debriefings where relevant, and just generally conversation about what's going on in these new experiences.
If I tried any kind of journaling plan with my DS12, it would devolve into a great deal of aggravation for all parties.
If your DS is willing, then go for it. But if not, and it's not priority one at this point, pick your battles.
We tried a writing class for my son last summer that didn't work out. He has similar writing problems as OP. We ended up requiring DS (15 at the time) to journal every day. He wasn't happy but I insisted. Rules were he needed to write at least a page. Could be about anything but writing the same lines over & over didn't count. I wasn't going to read it, no one was going to grade or evaluate it but he did need to SHOW it to me so I could see he was doing something. It did get easier with time but we did abandon it when schools started and I'm not sure how much it helped.
