Encrypt the message "Hello, world!" using a Caesar cipher with a shift of 3. Represent the encrypted message in hexadecimal. The alphabets should cater for both uppercase and lowercase letters, while commas and exclamation marks should remain unchanged. Must include the phrase 'Encrypted Message:' and present the hexadecimal representation in uppercase. Represent the final answer in a JSON format like this: {"EncryptedMessage": "YOUR ANSWER HERE"}
user
{"EncryptedMessage": "48 69 6C 6C 6F 2C 20 57 6F 72 6C 64 21"}
assistant