Do you usually get to decide about the picture? Coordinate the outfits, determine the setting, etc.? Because I usually do. ;) I spend a lot of time and energy deciding on exactly how I want to picture to look. I can get a little crazy about it, truth be told. I just want the "perfect picture", is that too much to ask? ;)
Anyway, maybe this is your DH's way of saying that he would like the pictures to be a more collaborative effort? Or maybe he doesn't like the stress of having to stage the perfect photo? Maybe he just has a different idea of what makes a picture "perfect", kwim? Can you compromise? Maybe he would like a fun, "real" picture of you all just relaxing at the beach, sweaty, sandy, wet, and all? I think that would make a beautiful picture. I think I would wait until "the golden hour" for the best light, if possible. But I think you may be surprised at how beautiful a relaxed picture can be. At the end of a long day at the beach there is usually such a feeling of togetherness and contentment and the spell would be broken by going home to change. IDK how long you are there, but could you do a "just as you are" photo at the end of the day at the beach and then a "cleaned, dressed, and groomed" photo at the beach the next day (morning might be easiest for this shot)? You have a lot of time to decide which to use for the card. Or you could use both with the many collage options there are now.
Just some thoughts. ((hugs)) I hope you can work it out so that you each feel heard and have your needs met.
ETA: This may not be about the photos at all. Maybe he is feeling a lack of control in some other area of life? Or maybe he is not feeling heard and validated. I would try for a compromise about the photo first, being aware that it may open up other feelings.