A few sessions ago, my Paladin (Oath of the Ancients) complained that his class was the weakest. I reassured him about its strengths and the things he just did, and he seemed open to learning. However, since then, he constantly interrupts the game—over-explaining his Oath, repeatedly using abilities out of place and commenting mechanics that aren't actually happening like tremor sense or divine sense. One patient player even said, "Can I speak? I've tried four times and keep getting interrupted."

Well...

I set up a special boss fight to highlight the Paladin and gift him a magic sword. A friend guest-played the boss, and he wasn’t an expert, so I helped when needed (as always with my players). The boss had three phases:

1. Phase 1 – Vulnerable to physical damage, immune to magic.
2. Phase 2 – The opposite.

Players quickly figured this out. However, during the battle, the Paladin frequently interrupted my guest's narration with questions he was about to say. This frustrated him, who responded, "I'm trying to say it..."

When the Paladin used Divine Smite, my guest (unfamiliar with the spell) said it was blocked, the paladin got upset asking why, my guest asked if it was a magical blade from the sky or something. The Paladin couldn't explain (he uses it since lvl 1), leading my friend to say, "If you don't know, how would I?" I clarified: the weapon struck effectively, but the magic was nullified.

On his next turn, the Paladin attempted to snare with magical vines, which failed due to the boss's resistance. He imagined it wouldn't work and declared himself useless, and refused to fight—opting only to heal himself each turn he even congratulated me on creating the boss, but I felt it was sarcastic. The party explained he was crucial for this phase, but he ignored them. The fight dragged on, with mages resorting to daggers in Phase 1. Other players grew so frustrated they threatened to attack him if he didn’t help. The session ended early due to this tension and everyone was unconfortable.

How do I handle a player who frequently interrupts, refuses to engage when things don’t go his way, and ignores his strengths despite encouragement? I aim to keep the game enjoyable for everyone, one of our favorite players is his sister (I don't think she would keep playing if he's not) and he's even our friend and work with most of us every day.

Edit: The guest wasn’t part of the original plan — it just worked out that way because a friend of mine really wanted to play and happened to be here that week. So yeah, we play in person.

Edit 2: Phases 1 and 2 were supposed to be easy, especially for the paladin, because it's versatile. The third phase is where things would get harder for them, with no gimmicks, just raw power. From the moment they arrived on the battlefield, a voice was speaking to the paladin, asking for help. In this phase, he would finally be able to discern where the voice was coming from. It is a cursed greatsword and after a trial tied to his backstory which he has to judge the innocents and the sinners he'd free the trapped souls and obtain a magical holy sword, which would really hurt the boss.

Edit 3: Today, he barely spoke to anyone, even to people who aren't involved in the session. I asked if we could talk after work, but he suggested we do it tomorrow since he had to leave, so I'm going to think about every single advice you gave, thank you so much.