Direct sunlight can heat a dobe up quite a bit even when the air temp isn't that high. They don't have any undercoat to protect their skin...whatever temp his outer coat is in the sun is what it will be right up against his skin. And my black dogs got almost hot to the touch in the sun.
If you're starting in the morning, the sand will be as cool as it's going to get. I figure that if you can walk comfortably in the sand barefoot (without feeling like your feet are burning) he should be able to too. Check the sand with your hands too to keep track of how hot it is getting. But remember that one of the major places dogs sweat to keep cool is through their feet, and dogs aren't as efficient at keeping cool as people are. If you're feeling hot, he may be quite uncomfortable. It would be a good idea to get him used to booties and have him wear those on the hike, just so his feet aren't in contact with that overheated sand.
Sand heats up, and is also more tiring to walk on than an ordinary packed dirt trail. Be prepared for a more strenuous hike for the distance than you would expect.
Bring along extra water, in a water bottle or two, not just for him to drink, but so you can wet him down from time to time...his ears, tummy, areas where his fur is thin, and where there is a lot of blood supply to his skin. And also along his back where it is hot in the sun. It might even be a good idea for you to dunk his feet in a little water from time to time if you feel like he's looking uncomfortably hot. You could also bring along a plastic baggy with a few wet washcloths in it---store it overnight in the fridge, or even put some ice cubes in it before you start, just so you can cool him down more easily.
Hiking and dobes go well together, but they do seem to poop out in the sun fairly quickly, in my experience. Test him out on walks around your neighborhood first to see what kind of temps and distances he's comfortable with before heading out into the "wilderness".
And bring your cell phone.