Because of this horse’s age, I think it is safe to assume that his teeth aren’t what they used to be. For that reason, I would probably soak the cubes.
For healthy horses with good teeth who do not bolt their feed, I have never found soaking to be necessary with alfalfa cubes. Many horses don’t like soaked cubes, and it can be hard to keep them from spoiling or freezing before they are consumed when large amounts are fed. I studied in grad school under a researcher who is probably one of (if not THE) the most well-respected equine nutritionists in the world. The vast majority of horses on our studies were fed dry alfalfa cubes as their sole source of forage. No chokes yet, and we fed everything from weanlings to geriatrics.