The kind of steroid shot you are talking about is short acting and temporary. It would not cause the kind of problem you are dealing with. Please get him on a good quality of grain free dog food. I feed raw but there are a couple of decent kibble foods out there. Keep him away from the people food. Buy some or make some quality treats and give only on a limited basis. I would not give any treats until he is totally well.
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Be careful even buying things that are labeled "all natural." Read ingredient lists. If you can't identify an ingredient easily then don't use the product.
Look for companies that only use grass fed stock that have not been injected with steroids, antibiotics, or hormones. Sorry to say that most of the meats being sold for human consumption do have these items in them.