'Gooner' has two completely different meanings. The traditional one is a fan of Arsenal Football Club ('the Gunners' → 'Gooners') and is harmless. The modern, much more common online meaning refers to a man who compulsively consumes pornography for long sessions, often in a self-deprecating internet-humour context. The two senses share a spelling but nothing else. If your teenage son uses 'gooner' in football chat, fine; if it appears with other internet slang, it likely carries the second meaning and is worth a calm conversation.