I've actually helped a number of guys write coming out letters if you get to a point that you feel stuck and want someone to look it over you can DM me.
Some general notes though.
• Write confidently avout how you feel to avoid confusion. For example avoid writing things like "I think" and "maybe". This is who you are and what you need from them and for yourself.
• Provide resources. Helping guide them in expanding their knowledge will benefit you both. PFLAG is one of many great starting places where they are able to hear experiences from the prospective of other parents.
• Ask yourself what do you want the outcome of this letter to be? Do you want them to use different pronouns, a new name, help with gender affirming treatment, etc?
• Come up with an action plan based on those answers.