Not sure if your vet will issue you with a detailed after op care sheet?
The most common problem with casts on legs is moisture.
Even a tiny drop of moisture can cause the foot to rot so make sure his leg is well wrapped in a plastic bag each time he needs to go outside.
No stairs, no jumping etc.
When you get him outof the crate, snap a short lead on him immediately, as most pups will want to try to bolt.
Probably best to gently carry him and place him on the ground for a few weeks.
Luckily hes still small! :D
Crate rest is 100% vital to the fracture healing correctly.
I had a puppy similar age and she was crated for the better part of a year while her leg and tendons were healing.
I had a trolley crate and wheeled her everywhere I was.
She even went for a "walks" in her trolley round the park!!!
We all survived it ....just!
My advice is lots of toys , raw meaty bones, lots of attention, some out of crate cuddling but NO play!
Also keep him crated in the hub of activity so he sees everything.
His leg will heal quickly if its rested and soon he will be back to gentle play then onto normal activities.