In my field, that's done all the time. It isn't unethical, he has to read it and sign it. You should give him an electronic version, and he should edit it, sign it, and seal it - so you will never actually know what ends up in the letter. That's standard practice in jobs like this.
Don't worry, write yourself a nice letter and count on him to make it his.
In fact, I would write the letter first, before asking a collegue/boss for a letter, give them the letter (electronically) for them to do with what they want. Most people in that position are too busy to start from scratch, and that gives them what they need to know - relevant background about you and the position you want, so they don't write a generic letter or a letter that suggests you are a good dog walker when you want them to think you are a good cat wrangler.