My DS6 is also skipping 1st and going into 2nd. Academically, we are not doing anything over the summer to prepare for 2nd unless DS wants to. Keep in mind I'm acting on zero experience with skips, so although this is what my gut tells me to do (or to not actively do anything) in DS's case, it's not a recommendation, just another point of view. My thought is that 2nd will quickly become underchallenging for DS so if he has the opportunity to have a normal (or even steep) learning curve at the beginning of the year it will be good for his student skills. Grade-school curriculum is "spiralled", so they generally do a brief synopsis of what is taught in the prior grade anyway before introducing new material.
In terms of social stuff, he is attending a small summer camp where all the ages are grouped together so he can interact with older kids. He is at least average height and I think he should be able to hold-his-own athletically.
I casually told DS that we (mom, dad) and the school thought he'd learn best in 2nd. DS has voluntarily told his classmates in a matter-of-fact way and the kids haven't made a big deal out of it (one benefit of doing the skip early). The only ones making a big deal are other parents! At DS's school, the parents know next year's teacher so I tell other parents if they ask which teacher we have. I try not to let the icy silences and ignorant comments get to me (e.g., yesterday a camp mom said, "Did you think he was too smart for 2nd? My daughter is reading at an 8th grade level and I would never skip her to 4th because I want her to have time to be a kid, blah...blah...").
We'll see what happens!
Jool