hey there
when i first got my boy i always played with him in a large area (fenced in!!), either indoors or outdoors (which he loves the most) where he could run around freely.
it might sound silly, but dogs respond quite well when you use almost like a baby talk and speak to them in a high pitched voice in order to excite them and transmit your playfulness to the dog.
i have also learned from experience that dogs like it when you get on the floor with them so you are on their level when you are playing with them.
i hope this helps! its possible that he came from a home where the owners did not play with him too much. glad to hear you rescued him!!
