Thank you for your patience. There are a few things that could be going on here including gastroenteritis, to a foreign body issue where the bones of the fish head are scraping the walls of the intestine causing irritation and bleeding, to an infectious issue (especially if the fish head was not fresh and could have been harboring bacteria).
The fresh blood in the stool you have seen is likely due to colitis (inflammation of the colon) and this can occur either due to irritation from the fish head scraping the intestinal wall, or simply from the sudden change in diet (trout is quite oily as well). If there is a lot of fresh blood coming out of his rectum, or being passed with his stool, please get him along to your local vet or ER vet today if you can. If there was a little bit of fresh blood in his stool, then starting your boy on a bland diet asap will hopefully help to settle this. Cooked, boneless, skinless chicken breast and boiled white rice is ideal for this, and you can start him on this tonight. Please stop giving the bread and milk as this may potentially make things worse (dogs are typically lactose intolerant).
If you notice your boy's appetite isn't as good over the next few days, if he has any vomiting, or if he seems constipated, then please get him seen by your local vet as a foreign body obstruction would be a concern here. An xray and/or ultrasound may be necessary to rule out an obstruction here.
Best of luck with your boy and please do continue to keep an eye on your boy's bowel movements and appetite. I hope all of the above makes sense? Please let me know if you have any other questions. My goal is to provide you with excellent service – if you feel you have gotten anything less, please reply back. I am happy to address follow-up questions. Thank you for your business and I hope to work with you again soon!
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