My five year old rescue is skinny!?

I would definitely take him to the vet for a checkup if it's not been that recent since his last one. I agree with the other answer that him being skinny could be alot of things, but my concern is the dalmatian/pit mix being laid back. seems a little out of place with that breed type. both breeds tend to be energetic and geared towards the athletic side. seems like something else might be at play here.