I am sorry to hear you are worried about your little Maltese. My name is Rebecca. I am a veterinarina and will do my best to help.
A hunched back can indicate back pain, such as a bulging disc or pinched nerve. Is he more reluctant to jump onto furniture, or go up or down stairs?
If he is eating, and not vomiting, you could see if aspirin helps at all. How much does he weigh? You can give 1/2 of a low dose or baby aspirin if he weighs 8 to 10 pounds. Give it with food, do not use anything except low dose/baby aspirin, and stop immediately if he vomits at all.
If this continues, I would consider having an Xray of his back taken. If aspirin does not help and he continues like this, he may need pain medication that you can only get from your veterinarian.
Let me know if he is eating, if his gums look a normal pink color (or pale), and what else I can answer or help with.