Hi and WELCOME to YT
!
Yes, you should absolutely be concerned if your formerly playful pup suddenly changes and his behavior is very quiet. At this extremely young age, it means you need to get him to the vet ASAP bc you don't know what you *could* be dealing with.
If his appetite is down, that makes it a true emergency. When pups are this young and something goes wrong, they can go down very, very rapidly.
Please let us know how this pup is doing!