Help,my moms puppy is depressed.?

My moms new puppy is 16 wks old.My niece and nephew were fighting over him 4 days ago and they dropped him,he fell on his head.I took him to the vet,he said that Buster had a mild concussion and moderate depression from the hit.He gave him a shot and sent us home.Buster use to be the happiest puppy,loved to play and run around.My boyfriend even tought him to play catch and they would play for hours when I would puppy sit for my mom while she would work.Buster even loved to play with my cat Ryleh,they would play king of the pillow or tug of war with his Mr. Winnie.Now all he does is sleep.He don't eat much,he wants to be held all the time and may only play for 5 to 10 min. a day and thats while he is sitting on your lap.He doesn't run around,play catch,king of the pillow or tug of war and he won't even look at his toy box.His eyes are always glassy and he looks so sad.I am always hugging and kissing him and telling him that I love him.When he's not at my house we go to my moms to visit him.I hate that he is so sad.How long will his depression last and what else can we do other than giving him extra love to help him.We try to get him to play but he just don't want to.He just looks at you with those sad eyes and wants to lay in your lap.Please Help.