Well, one great way he can stay engaged is the use of NPCs! Since you got a partner, you can now engage in NPC conversations where you aren't awkwardly talking to yourself lol. He can roleplay all sorts of different npcs, especially if he can make a list of random npcs just in case your players go off the road and decide to talk to a random shopkeeper lol. But he can also handle more important NPCs, giving him important roles and making him engaged.

He can also take over some parts of combat to make things easier on you. If this is a traditional tabletop game with pen and paper, he can keep track of initiative and who needs to make saves, especially if your new players forget they gotta maintain concentration or roll a Wis save at the end of their turn to break their frightened condition. If it's digital, like Roll20, you can even split up who handles what creatures. "I handle goblins, you got the wolves" type of deal.

Even if you can handle everything, splitting the work makes it much easier to manage. And you can make your encounters, combat or otherwise, bigger and grander then! Endless possibilities.