There isn't probably isn't much difference for him being clergy or not if employed by army reserves. The main difference would probably in regard to housing. He assume he will not be "opposed to certain public insurance for religious or conscientious reasons." He will pay Federal Taxes and Social Security, just like for any other occupation. See the link at the bottom for more information.
Housing AllowanceA licensed, commissioned, or ordained minister who performs ministerial services as an employee may be able to exclude from gross income the fair rental value of a home provided as part of compensation (a parsonage) or a housing allowance provided as compensation if it is used to rent or otherwise provide a home. A minister who is furnished a parsonage may exclude from gross income the fair rental value of the parsonage, including utilities. However, the amount excluded can't be more than reasonable compensation for the minister's service.
https://www.irs.gov/taxtopics/tc417