Q: "How do you turn a boy into a Mermaid?" A: "becoming a mermaid is now impossible today , enchanting things and spells know better that to get into a humans hands."
Nope. You just cut off his dick, testicles and legs. Then you catch a big fish and you transplant the tail. Then give the fishboy some female hormones and breast implants, and perhaps some additional cosmetic surgery, and voila, you got yourself a mermaid.