Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
I don't think that your fellow is constipated. When the large intestine is irritated enough that it bleeds we will often see straining and the feeling of urgency even when there are no feces left to pass.
Of course anyone would be concerned about blood in the stool. Was it fresh, bright red blood or dark, tarry blood? The fresher it appears the closer the source is to his rectum.
The important factor in all of this is how Buddy is feeling otherwise. If he feels well otherwise and is bright and alert then that is a good sign.
Colitis or inflammation of the large intestine, can be due to stress, a change in diet, parasites, inflammatory bowel disease or even a food allergy. Rarely a mass in the colon can be the cause, or exposure to anticoagulant rodenticides. Another possibility is an anal gland infection. These are scent glands that are located just inside the rectum.
A raw, irritated colon can take a few days to heal. Do you know if he got into anything that may have started all of this? If he is feeling well and the blood is less than the original incident I am less concerned but it should clear completely over the next 2 to 3 days. If not he should be checked by his regular veterinarian and his anal glands should be checked. Well worth checking at least a couple fresh stool samples too as parasite eggs/cysts are shed intermittantly and they may be picked up on the second sample or third test if the first is negative.
If he still feels well though we can try some things at home now.
If his appetite is off and his stomach is gurgling he may have some stomach upset and reflux that can go along with his bloody stools.
To try and settle his stomach today you can try either:
1) Pepcid-ac (famotidine) at one 10mg tablet per 20 to 40 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at one 10mg tablet per 20 to 40 pounds of body weight every 24 hours.
These will reduce stomach acid and should help if this is related to simple nausea and gastrointestinal irritation.
I would not feed him any food for 24 hours after the acid reducers are started. This should help stop gut spasms and restore normal gut motility. Small amounts of water or ice cubes given frequently are fine as he needs fluids after all that he has lost with bloody stools. You can give him pedialyte to replace electrolytes too but Gatorade is much too high in sugar which can make his colon irritation worse.
Today even with the fast you can start Kao-pectate at 1ml per pound or 1 tablespoon per 15 pounds of body weight every 6 to 8 hours. This is quite safe and will coat his irritated gi tract as well as absorbing bacterial toxins. You can use it for several days until his stools look normal. You can find kao-pectate at the drug store.
If he has a tense painful abdomen, continues to have diarrhea after being on kao pectate for 24 hours, becomes very lethargic, begins vomiting or runs a fever greater than 103.5F then he really must be seen by a veterinarian as soon as possible. Make sure to take a fresh stool sample with you when you go.
After his food fast start a bland diet of 1/2 boiled hamburger (or white skinless chicken), all fats and juices drained off mixed with 1/2 boiled, plain white rice. Feed several small meals a day. You might wish to add 1 to 2 tablespoon(s) of canned pumpkin to each meal as fiber helps soothe an irritated colon.
Once he feels better (no bloody diarrhea for 48 hours) start mixing in his regular dog food very slowly. Less bland more regular with each day. It should take a week or so to convert him back.
If Buddy begins vomiting and feeling poorly though it would be best that he see a veterinarian now as anything you give him orally will just come back up worsening his dehydration.
Best of luck with your fellow, let me know if you have any further questions.