Staffys tend to be pretty bulky dogs naturally anyway, so I'm not sure you should really be doing anything to bulk it up further. Maybe you just need to wait for it to grow and mature to its full size?
Secondly, this type of question does remind me of the kind of person who sees their dog as an accessory or a trophy, when really, dogs can be very loyal and affectionate companions.
Anyway, if you're determined to bulk up your pooch, here's a really simple how to guide:
How to bulk up your Staffordshire Terrier
1. Feed him well - Protein builds muscle mass, so you'll want to make sure your pet is eating plenty of meat. A balanced diet is vital though, because a healthier Staffy will be more active which brings me to point two...
2. Exercise - Just like a human, dogs need to exercise to build their bodies. Maybe a dog aerobics class would be a good idea? Or just a run in the park. The main thing is keeping active and motivated.
3. Mental stimulation - This might not sound like it'll have any direct impact on the way your dog looks, but keeping a Staffy entertained and motivated is actually pretty important because it helps points 2 and 3.
A happy and alert dog will be willing to go that extra mile (literaly) in terms of exercise, and it will also have a healthier appetite.
Try to imagine how you'd feel if you were mistreated and down-in-the dumps... You wouldn't want to go for a jog and a big meal, would you?
Finally, sometimes it just comes down to genetics and metabolism - things you can't do a whole lot about!