Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 15 years of experience. I look forward to working with you.
Before I discuss supplements and supports you can do, it is important for you and your vet to search for the source of the anemia. Lack of a spleen will not cause anemia. Yes, this was the warehouse or storage area for red blood cells, but the body compensates for its lack. So, you need to work to discover is he anemic because his body is not producing more (and then why is this since not normally related to a heart murmur) or is he having a slow bleed from somewhere? Has he had an echocardiogram to see if he has a tumor around his heart? This is a very common location for a hemangiosarcoma in a Golden. Therefore, the first step is to know why he has anemia.
If not done already, then you should start him on an iron supplement. You should get this from your vet because not all iron supplements work in dogs, and many over-the-counter can cause nausea. But I know your vet has an effective iron supplement, usually in-hospital.
From a holistic perspective, you may want to consider switching him to a homemade diet. And why? I do not know why it works. But I can say that after many years of changing dogs from commercial diets to homemade diets with the majority suddenly getting healthier, it makes intuitive sense to use this as a preventive strategy. To do an effective homemade diet means that it needs to be balanced; so you need to get a good book that has recipes. I am a huge fan of Donald Strombecks Home-Prepared Dog&Cat Diets: The healthful alternative (amazon UK link: http://www.amazon.co.uk/Home-prepared-Dog-Diet-Strombeck-Donald/dp/0813821495/ref=sr_1_1?ie=UTF8&qid=1405252142&sr=8-1&keywords=donald+strombeck+home+prepared).
Another good book is Richard Pitcairn's Natural Health for Dogs and Cats (Rodale Press): http://www.amazon.co.uk/Pitcairns-Complete-Guide-Natural-Health/dp/087857395X/ref=sr_1_2?ie=UTF8&qid=1405252204&sr=8-2&keywords=Richard+Pitcairn%27s+Natural+Health+for+Dogs+and+Cats
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