Good stuff, glad to hear it's all coming together with Billy.
Same as doors, don't let him on the stairs until you've used them either. Make him sit and wait, top or bottom as required. They usually pick that trick up pretty quickly and it helps with the 'stay' command.
Jet's 38kg, but he's a Czech type shepherd. The old DDR patrol dogs. After 50 years of selective breeding they ended up mostly black and sable and very high energy. They're slightly longer and shorter than a regular GSD to look at and they mature more slowly. Swimming helps me to wear him out, for a few hours. I'm feeling fitter now he's 14 months old and, if I still worked dogs, he'd make the grade quite easily.
He does have some vices though.. He spins like a whirling dervish when my grandson comes around (although not when I'm at home). My wife had to video him doing it as I said she was making it up. It's a wonder he doesn't puke. Just as well that I bought a cage for such occasions really.