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Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
It sounds like you're actually seeing signs of colitis (inflammation of the colon). This leads to soft or liquid stools and the constant urge to go. That would explain the straining.
If it were true constipation, there is not usually anything that comes out and it's often due to dehydration. I would definitely try to get fluids in him. The other thing that can help, is extra fiber in the diet. You can do that by giving 1-2 teaspoons of canned pumpkin or sweet potato baby food every 8-12 hours.
I hope this helps, let me know if you have any other questions.