Answer:
Sylvester insisted that his friends call him Syl. His parents. However, continued to use his given name.
Step-by-step explanation:
If "however" is used to join two independent clauses you should put a comma after using the word. Putting it before is a gross mistake.
"However"" is used to introduce a statement that contrasts with or seems to contradict something that had been said previously.
I hope this answer helps you.