Hey there, My son is a IS and was at Dam Neck at this time last year. He did get to come home for Christmas, but he finished A school on 12/20 came home on 12/21 then went back on 12/28. Some of them went home Christmas week then the others went home New Years' week. C School started the week after New Years'. The timing of A and C school was good last year for the holidays. The school will be close for a couple of weeks during Christmas, but that doesn't mean that they will approve leave.
We went to Dam Neck to visit 3 times while he was there, but I would advise you to wait until your son is in C school to visit. If you go during A school, you will not get to spend much time with him. It is much easier to visit during C School. My son is VERY happy with his rate and says that IS might be the best rate in the NAVY.
I wish your son the very best! Tell him to hang in there, C school is much better than A school and it will be over before he knows it.