Problem is, Thor, that most of the services won't do ASVAB/med and then DEP in at a later date. It's all one big ball. Are you saying they won't DEP in 11S for Air Force, only 12L (workforce)?
But you are correct--what's available is better in the winter months--SOMETIMES, based on ship date. Hate to tell you, but if he's going into a small field, you may not get him to ship right after graduation.