Hi Michelle! My son too arrived yesterday. My thoughts are with you!
Yes, the midnight call is what we refer to as the "I'm Here" call. If he left a message then you know it was a very scripted sounding call. When I received my son's I thought it was an automated call. I just kept saying I love you, and barely heard what was said. I did not find this website until just right before his PIR, so I pretty much relied on the information I could glean off of the RTC site for what came next.
Be sure to read the information on the BC Mom main page. There is a lot of information there. Some things have changed since my son went to BC but you should receive your son's box with his clothes, belongings, and cell phone. Honestly, I am home a lot, so I don't even know whether Fed Ex delivered ours or not, but that is what I hear presently, and you have to sign for it. If you are not home, they should leave a slip for you to sign authorization for them to leave the box.
The "Form" letter arrives later and now, I believe they have to pass their physical first before that is sent out, because it will have their training group and division listed as well as their graduation date. We do not recommend you send any mail to your recruit until you receive the form letter. However, you can start writing letters to your son beforehand. Date them and have them ready to mail when the letter arrives.