What is more important to help a dog gain weight, higher protein or higher fat content?

I just recently purchased a high quality registered APBT and through his adapting to a new environment he appears to be experiencing some weight loss. Im not sure if this is due to the summer and the heat causing him to lose weight so I was thinking about trying him on a higher quality kibble but I was wondering in looking at dog food what is more effective in weight gain, higher protein or higher fat percentage?