My fiance normally wears an XXL or an XL depending on the brand of shirt, and I ordered him a L. The sizes run really big, and it's only slightly snug on him, but we're both losing weight so it should be good by July. (The shirts are long, so they don't just get wider with size.) It's just a little tight around his tummy, so if your father doesn't have a large tummy a large should work for him.
Just an FYI: We also ordered the bobble head shirts because they were only $4.99 and I got him an XL and it's a little big.