My Dog Needs Help! He gets over exited and nips!?

We have a dog and his name is Cooper and we love him to death! He is sweet, very lovings and very caring But he has many many issues. When he was a puppy he was abused by his first owners and was kept in his cage and kicked and hit by his owner untill he was 1 1/2 then a girl rescued him from that proublem. Then we got him and now he is 2 and very healthy and is small with a big head and BIG LONG TEETH and there begens my problem! He gets very exited and nips. He also has issues with my older dog Sammy,and Sammy is not Neauterd he can't be, but Cooper is. But Cooper wants to play with Sammy and Sammy does not want to play and that causes a fight (thats only when cooper gets exited). Cooper gets very exited I mean very exited, And I am not getting rid of him so we need help to find out what going on with him!