When taking a horse's temperature, it is a good idea to comfort him first. Chances are that if you are taking a horse's temperature, he is already not feeling his best and may be stressing due to an illness. Some horses also get nervous if they sense something is not right, or when people are behind them.
Talking to your horse, petting him, and even offering him one of his favorite treats will help to put him at ease before you take his temperature. It also will help if your horse if familiar with the person who will be helping you with this task.
As you begin the process of taking his temperature, keep talking to him. This will comfort him by letting him know that you are still there with him.