This is how I imagine what happened after Tommy left:
Buck locks his door, grabs the trash bags he needs to bring downstairs, and almost runs right into Ms. Elsie, his neighbor who had taken over when Veronica had moved out.
(And hadn't that been a glorious day, coming home and seeing the movers running from her place to the elevator? Buck hadn't even cared about the sneer she sent him.)
"Oh, Buck," Ms. Elsie says, and he smiles at her and bends down so she can fuss with his hair, like she always does. "Who was that dashing young man last night?"
And Buck... look, it's not like he had forgotten Tommy had been there - impossible, he'd never forget that - but he hadn't expected anyone else to know. Somehow he had been sure Tommy's visit had happened in a vacuum, just for him.
"Uh. That... you saw him?"
"Honey, I was on my way out, there was no way I could miss a strapping gentleman like him do a fist pump in front of your door and more hop than walk all the way to the elevator."
Buck's brain needs a moment to parse through what she is saying, but then a giant smile breaks out on his face, realizing that Tommy had reacted like that to Buck agreeing to a date. After their kiss.
"Oh, I see," Ms. Elsie hums, a knowing smirk on her lips, and, really, she should be too old to look this mischievous. "Well then," she pats his cheek, "enjoy your time with your boyfriend, Buck!"
She walks into her own apartment, leaving Buck standing there, frozen, the word "Boyfriend" running through his mind on repeat.