We can do some things to ease stomach upset and help if this is back pain related.
Since it has already been more than 2 hours since he possibly ate the chocolate it would be out of his stomach and it is probably too late to induce vomiting.
I would encourage you to get him to drink as much as possible and to get him out to urinate frequently. Chocolate toxins are metabolized out through the kidneys and can be reabsorbed from the bladder so we don't want them to sit in his bladder for long.
In most cases mild chocolate toxicity is out of their system in 24 to 48 hours. We generally begin to see symptoms, if they eat a toxic amount, within 4 to 6 hours.
If he seems at all nauseous it may help to with-hold food for the next 12 hours to stop intestinal spasms.
To try and settle his stomach you can give either:
1) Pepcid ac (famotidine) at a dose of 1/4 of a 10mg tablet for a pup 5 to 10 pounds in body weight every 12 hours
2) Prilosec (omeprazole) at dose of 1/4 of a 20mg tablet for a pup 10 to 20 pounds in body weight every 24 hours
These are acid reducers and should help him feel less nauseous. They are quite safe and can be used for several days if needed.
Then after his 12 hours food fast to decrease the symptoms of stomach upset you can feed him a bland diet for the next couple of days. A homemade bland diet is a mix of 1/3 boiled, minced, white skinless chicken or lean boiled hamburger and 2/3 boiled, white rice mixed with some low salt chicken broth to make it easy to lap up and swallow. Feed small meals frequently.
If you do notice a very rapid heart rate (more than 160 to 180 beats per minute in a small dog) repetitive tremors, or seizures then he should be seen by a veterinarian on an emergency basis for treatment. They can use injectable medications to control his heart rate and stop the seizures until the toxic levels fall.
I would keep him very quiet today. No running, stairs, or jumping on furniture allowed.
Leash walks to go out both to keep him from running and to watch his eliminations closely today.
Warm packs on his back (either a heating pad on low or one of the microwaveable hot packs) for 10 minutes at a time several times a day to decrease painful muscle spasms.
Then watch him for further symptoms.
If he starts vomiting and cannot keep anything down, seems to be getting weaker in his rear end or uncoordinated or refuses to eat tonight then he should see a veterinarian as soon as you can have him seen. I understand that may not be until tomorrow morning given it is a holiday.
Please let me know if you have any further questions.