Suzanne, first of all, I would like to invite you to join another group on this site - the Beyond BC group. There are a couple of moms who had sailors in Pensecola and others who have mailed items to their sailors, who might be able to help. One of them also replied on the BC Mom group page down in the comment wall.
If your sailor is still at PIR, then if you are able to attend, you can take his carry on and meet him at the airport for him to have checked with his baggage. The airlines usually allow extra baggage for military flying on orders.
If he is already at Pensecola, then I highly recommend that you do not mail his laptop inside a carry on. Agnes B posted that she shipped her son's laptop via UPS. You will want to make sure it is packed well and also insured with tracking!!
Clothing can be shipped via USPS via the Priority boxes. Military Care Package Kit | USPS.com - You may also go to your local post office and pick up the boxes. Ask for the flat rate boxes, that includes tracking. You will want to find out from your sailor when his mail dept. is open for him to pick up his boxes. My son was not in FL for A school, but I always planned to mail my packages when I knew that he would be able to get them picked up from the mail room after his classes. Most likely the mail room is open M-F until 5 pm, and on Saturday until noon.
I don't know what your sailor's rate is but for my son they were not allowed electronics in their study area and he only used his computer for personal usage, so this may not be as urgent. Also, with the USPS, if you are shipping anything with Lithium batteries, they might need to mark those specially.