I've had my rotty boy for 12 months now but prior to my ownership he was abused and neglected and severely underweight. He is also quite short compared to others his age. Is he not putting on weight because of the condition he was subjected to being in from his former owners or is there more I can do? My staffy is in perfect shape

All dogs are different, so their weight and height really just depends on their genetic make up. First let me commend you for rescuing! In the mean time it's important to know that gradual weight gain over a longer period is what you need to do. Sudden eight gain can cause issues with joints and ligaments and also cause pancreatic issues. As long as you are feeding a good quality food, he is up to date with worm and flea treatments, he WILL put on the weight. A leaner dog usually always has less issues with hip dysplasia and other joint issues.

What are you feeding him now? How much does he weigh? What is your vet saying?