Comment by spectraldrift

This sounds incredibly frustrating. But given you're in the leadership seat, it's worth taking a hard look at yourself asking how you're playing a role in this, especially since it's unlikely people are this difficult for no reason. I am rather a bit skeptical of this account as written.

One hint: he was told he was meant to be the lead. That's a bit of a shitty promise to be given and taken away.

This guy is almost certainly operating from a place of "status injury." He sees you as the person who took his job.

So, some hard questions for self-reflection:

Knowing he felt slighted, did you ever try to build an alliance with him and acknowledge his expertise? Or did you just expect him to fall in line?

Are you showing him respect, or just demanding it because of your title?

Are his arguments over "trivial things" really trivial? Or is it his (unproductive) way of trying to assert the technical authority he feels you're ignoring?

Right now, your manager hears a personal problem ("This guy is a jerk to me"). That's why he's giving you the weak "don't let it bother you" response.

Stop making it personal. Use your leadership skills to actually lead this person. Try to fix the relationship. Give him ownership.

If that fails, you can go to your manager with a leadership problem ("I've tried A, B, and C to leverage his skills, but his behavior is still causing X business risk"). That is a problem a manager has to solve.