
Oooh I can totally relate to this one.
My male Stafford is exactly the same. Very much on the slim side, but still weighs in at around 19kg too. He's now just over 3 years old.
He's slightly longer in the leg than some Staffords you see, he isn't 'barrel' chested i.e. the roundness in the ribcage, but he's fit and he's healthy and happy.
He too used to be fed on AG when we were still in the UK, and that food suited him perfectly. I still added oily fish from time to time, and the odd bits of pasta or rice, to try and 'bulk' up as it were, but to no avail. He's got good muscle tone though, so I'mhappy with him.
The spring pole and also flirt pole will help with your dogs muscle tone for sure.
If you'd like to pm me I can supply a name for a great supplier of these items.
Be happy with him. I'm sure he's just lovely