A man's proper outfit for a morman service would be slacks and a tie. Usually dress shirts are worn.A jacket is optional but no tennis shoes. Baggy clothing is discouraged and above all the key is modesty.
Most men will wear black dress pants, a white collard dress shirt and a tie (color of your choice). Khaki pants and Colored dress shirts are also quite acceptable to wear. As for shoes, black or brown dress shoes are typically worn.
Proper men's attire is similar to proper attire for any other church service. Dress shoes, dress trousers, button-down shirt and necktie are always appropriate. A blazer or jacket is desirable. It's not necessary to wear a business suit, but it's entirely appropriate if you do so.
You should mainly dress nicely. A suit and tie would be nice, but you can also dress more casually if you like. There aren't necessarily any requirements, but slacks and a nice, button up shirt should be fine.