If it were my daughter and son I would have him in either white dress shorts with a matching button up shirt that’s the same color as the dress or dress shorts that’s the same color as the dress with a white button up and matching bow tie (color of the dress). Maybe a shoe and matching belt with beige and a navy blue. Looks cute in my head lol
Can you message the photographer? They often have great insight!
Maybe a pink button down shirt and khaki pants ?