To be supportive, just be the woman he married and loves. You don't have to say too much, just be there. As far as what you can say, just make sure he stays in contact with his recruiter. I wouldn't call or email every day... but once a week to 10 days, that would be acceptable. He wants his name to remain in his recruiters mind, he wants to show he wants this, is ready. Once he goes to MEPS things will move much quicker, so long as he qualifies.
While he is in boot camp, yes you can write. And you should write and write and write and write!! He might not get a chance to read everything but knowing you are there with him in spirit and cheering him on, it is a great morale boost. Keep the letters to a page, front and back maybe, but not anything more than that. He will have very limited time to read and even less to respond. Only expect a return letter about once a week. When we get closer to shipping we'll share some secrets on sending the "right" letters, even though no letter is wrong. Boot camp will be the hardest stretch of separation. There is little to no contact from him to you while in boot camp, the occasional letter and only a couple of phone calls. So that will seem the hardest time. It is said, and it is true, the days drag by but the weeks fly past! I don't know how that is but it is true, Monday takes a lifetime but week 1 is gone in a split second. Has anyone in your family or his ever been in the military? If not, they won't get your loneliness. That's when you come here, you join his boot camp company facebook page, you join the CG spouses page, you join other Facebook CG Friends and family pages... Reach out to the people that have been there, that understand, that can lend a virtual shoulder to cry on. People with no military experience will say how lucky you are... you don't have to make dinner for him, you don't have to shave your legs, you can eat fruit loops for dinner when all you want to do is make dinner and cuddle. So try not to let that get you down. Once he is in the fleet, out of boot camp, you'll have other spouses at the unit and if it is a cutter, you'll make friends with other wives or husbands and you make your own fun with them. You become a part of the CG family and you will have friends you will have for your entire like. I am still friends with people from our first unit in the CG... that was 1985!
I've babbled enough... Whatever questions you have, ask away. There are many people on this forum that like to help. Best of luck to both of you and tell him to just keep talking to his recruiter and let's get this party started.