Field-grade officer Letter of Recommendation - what's the appropriate channel?

Help him help you. Email is ok, remind him who you are. Update him on your successes. Provide him the board address. Share your timeline and when you need it. I never said no to a request but always insisted on the Soldier doing some of the work. “To Whom It May Concern” (usually for civilian jobs) would have me say “Ok, get me a name, position of the hiring manager and address and I’ll have it done in two days.”

I always take it as a compliment when a former soldier of mine thinks enough of me to reach out for a recommendation. Just send him an email and remind him when/where you served under him and ask if he'd mind writing you a recommendation for OCS. Since it's only been a year, I'm sure he'll remember you and be happy to do it. point of reference, I once reached out to a retired GO whom I had served under as his HHC Commander when I was a 1LT and he was a COL. It had been 10 years since I'd had any contact with him and he was thrilled to hear from me and more than happy to help me out.

Bottom line: If your relationship was such that you feel he'd be willing to write you a recommendation for OCS, then sending him an email "asking for stuff" shouldn't be an issue. I've had numerous former soldiers contact me wanting a recommendation for something and in the 17 years I've been commissioned I've only ever turned one down.