Casdon
It depends how invasive his operation will be. My boy had this operation, but the vet was able to remove the undescended testicle without invasive surgery when he was neutered. Sometimes it is retained in the abdomen, which is a more invasive procedure, but I was told they wouldn’t know until they operated exactly what was needed.
My dog was fully recovered within a week, and he was able to go for short walks on the lead within two days. He’s very food orientated, so we used Kongs and other puzzle food toys to keep him occupied. He also likes TV programmes with dogs on, and birds and squirrels (he’s a spaniel) so he watched a few of those too.
This happened to one of mine, the retained testicle being found relatively easy, although his poor penis was cut one end to the other. It didn’t bother him and was short lead walks within days. Bless him he was only five months old, much too young, but the vet said the retained testicle would have caused him hormonal problems due to it overheating inside his body cavity 
