Use sandboxx (google it) to send letters. It’s a website. You type the letter and he’ll get it a day or so later, and you get to add one photo per letter. Also includes stamped return envelope and paper. It’s like 1.50 a pop. Totally worth it. My brother in law graduated Sill last month and that’s how I sent him letters. It worked awesome, and was painless.
The army is weird and does block leave, even from BCT for the holidays. The USMC didn’t. I think policies probably vary by installation given COVID. I’m currently in TRADOC land at USAACE and we will have one week to quarantine/handle matters remotely before training comes back in full swing after the new year.