But if the five other justices write concurrences with different reasoning, does his opinion really count?
I believe so because he’s authoring the majority opinion and it’s the majority opinion, not the concurrent opinion(s) that have the force of law.
IOW, six justices can all reach the same decision. But the reason for that decision that is articulated in the majority opinion will be the only opinion that enjoys binding precedent on future cases. The concurrent opinions could only - at best - be considered persuasive opinions.