I am afraid you are dealing with diarrhea rather than constipation. Dogs suffering from diarrhea as it happens with colitis, tend to develop diarrhea which starts in large amounts and then gradually, less and less liquid poop is produced. The dog may strain and just produce a bit of diarrhea each time. This is called "tenesmus." There may be mucus and blood mixed in the liquid poop. The foods you are feeding are likely making matters worse. I would recommend a vet visit since he seems to be uncomfortable and it sounds like he is in pain. Sometimes when a dog has repeated diarrhea, the anal sacs that are present on either side of the anus become full swollen, reddened and painful.