I have worked with several dogs who have had low food motivation so here is what has helped:
1. Decrease weight to get them to a nice "sports" weight. Even if the dog isn't considered fat by most people, there's a good chance they could still lose a few pounds and this can help increase that food motivation. At the very minimum make sure they're not free fed.
2. Use their meals to train. Not just have them sit or down and then give them the whole bowl, but actually train kibble by kibble. Of course in the beginning you want to keep sessions VERY short, so maybe only 5 kibbles and then give them the rest.
This has been huge in helping. I never met a dog that didn't get somewhat excited when they see their food dish, even if they don't always finish all the food (and are still at a nice thin weight). That excitement over the food DISH can transfer into excitement for training, especially if you keep the sessions short. Even my toller who's medications mess with his food drive big time and I can sometimes struggle to keep weight on him, he gets excited for the dish. Actually eating what's inside can be a different story
3. Competition. With the least food motivated dog I trained, I used my own dogs to increase excitement. I did 30sec of training with Mrs. Non foody, switched to another dog. And repeated that. Anytime the non-foody dog wandered away I let them and just switched dogs. Even if I didn't have another dog to switch with, if a dog wanders away I don't try and get them back. I might however limit their option for other activities by putting them in a kennel, or back inside the house, or just tethered a bit. It's not a time out, there is no anger involved, the dog just doesn't get to wander around the yard sniffing.
4. Other rewards. Toys are the easy one, but usually if the dog isn't very food motivated they don't care much for toys either. So I use sniffing as a reward a lot. Little tag your it games are also fun for a lot of dogs.