Hi what makes you say this? How old is he? When was the last time you had him weighed? Does he eat ok?
Assuming he is healthy, he probably needs more than just walks. Dogs need to run and perhaps swim. Unless you are a hard core runner, you can't walk or run enough to really get a dog tired. Cooper does not get as many walks as she should but she is lean and her muscles are like rocks. She gets a lot of hard running, swimming and other play.