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I started with chicken, turkey, and pork (his situation was a bit more dire, at the time I needed him converted like yesterday for health reasons). Chicken because it tends to be cheapest but also most safely packaged at local butchers/stores and same - he loves the PureBites chicken and duck (back when I could actually find the duck!)
I did a bit of an experiment with the same batch - I did some ground, some smaller chunks, then some slightly bigger to see what he'd go for. Some raw, some cooked (raw is better but again I needed him switch asap). Chunks better for dental health, but it's whatever hell eat to get him started.