I think he could have just gotten tired out. I suggest you try shorter walks - when he stops - go in a circle - walk slower - let him sniff. Also take him to different places - where there are other dogs and people .
Take him to places where he can explore a little as stated above. Let him do some sniffing and don't push the pace, make it HIS walk. Gradually, over time, you can set the pace but for now make it fun for him.
My dog doesn't like to walk either ... until her nose gets into the act and then we can move along. She is (almost) 4.
She does like to jog, though, and will stay right at my heel off leash no matter where we go.
I know my pups all of stages that they seem to not like to leave the yard. So I just use my yard for the daily poo/pees but for exercise I use my car.
By that I mean I pack the pup in the car, go to a NEW place (dog classes? socialization places) and that's where the fun and exercise takes place. When my dogs get older they are more secure and walking around the block is easy.