My husband who has dementia has gotten where he doesn't want to eat anything, even foods that were his favorites before. Any suggestions?

Depending on where he is in this journey you can deal with it 2 ways.
If he is still able to eat but is just "choosing" to not and drink Coke..use it as a bargaining chip. Just like you would a child. Eat a bit of dinner and you can have the Coke.
If he is more at the end of this journey and you just want to get some nutrition into him make the coke into a "Brown Cow" (my Mom would make what she called brown cows Coke or Root Beer and milk not ice cream, she called the ones with ice cream Black Cows) But in place of milk use a Vanilla Boost, Premier or other nutritional supplement.
Drinking is not the greatest way to obtain nutrition as there is a loss of the fork or spoon to mouth, the chewing aspect of eating. It is another slippery slope of decline that is not probably going to return.

Medications can make foods taste and smell differently.
Vision problems can make things look differently

also possible this is a phase and this will morph into another.
Gotta love the ever changing life of caring for a person with dementia.

It could be he is on a medication or has a medical condition that is affecting his sense of taste. This is a significant change in his condition and should be discussed with his doctor.

Angie, your profile says he is only 56 years old...does he have early onset Alzheimers? I'm so sorry, how difficult! Many on this forum have been in your exact situation and will be able to provide some suggestions for you. I love Coke, too, and I especially love a Coke Float (putting vanilla ice cream in it). Not the most nutritious thing but if you use high quality ice cream at least he is getting some dairy. Would he be willing to eat gummy vitamins like the ones for kids? If he likes orange juice you can make a "julius" by putting oj, a raw egg and some milk in a blender - comes out creamy like a Creamsicle and you can't taste the egg at all. Others will give you better ideas. May you have peace and rest as you care for your husband!