Congratulations!
My DS skipped 1st grade too and we did not do any preparation. He did just fine and actually enjoyed a learning curve for about the first 4 weeks of 2nd grade - after which he was right back at the top and a little bored! The only thing that was slightly challenging was that he was not use to writing as much as the kids who completed 1st grade. He didn't particularly enjoy writing, especially since his fine motor skills were not as developed. I guess that is something you could work on if you feel that this might be a similar challenge.
More than preparing him academically, I would prepare him for how other children might react to him. For months after he skipped, he would get asked 'why did you skip.' Formulating a good response is important. Some kids may be resentful and not particularly nice.
Plus, all his friends were now a year behind and the kids in his current class had known each other for 2 years and had already formed bonds. He now had to find a way to fit in. I tried to encourage play dates quickly and signed him up for cub scouts which included 3 boys from his 2nd grade class.
Hope this helps. Good luck
