I'll go ahead and give you an idea of what I think the problem is likely to be. Dogs are pack animals and there is always a boss or leader of the dogs. In most households, the oldest resident dog is the boss unless another adult is much more dominant then that dog. When a pup comes into the household, the older dog is automatically the boss. However, as a younger dog matures they start trying to raise their place in the "pack". In this case, the boss dog is smaller than the younger dog. The younger dog is most likely stronger as well.
With them not being neutered, both dogs are even in that respect. So the most dominant, strongest of the two will likely become the alpha of the dogs. Neutering a dog pretty much lowers them below intact dogs. Right now your younger dog is just becoming an adult and starting to challenge the smaller dogs position in the house. If he is jumping him now, when he is a bit older it will likely get worse and if the shih tzu fights back, you might end up with a pretty serious issue. Neutering the younger dog right now might keep the smaller dog as the top dog in the house and avoid a dominance issue between the two dogs. Neutering the smaller dog will lower that dog down to a lower position and elevate the larger do to the top position. Hopefully both dogs would accept that and not get into any fights as the leader changes. Sometimes the older dog won't accept the change and fights to retain the position. If it is with a stronger younger dog, then injuries may occur.
Your younger dog is not yet sure of himself so likely he is doing sneak attacks on the younger dog which is why he waits until the smaller dog is not looking at him to pounce.
There are a couple of options to stop the behavior. One is neuter the staffie now before he does any serious injury to the Shih Tzu and so he is not an intact adult male. The second option would be a lot of obedience training for him so you can control him at all times. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Additional training can also help. Both dogs should be leashed and if one dog even looks at the other dog, a correction should be done. Any sign of aggression including a prolonged look, hair raised on the shoulders, a growl or even a stiff legged walk, should be corrected. A correction is a quick tug of the leash and a firm low toned "NO". Once you have done this couple of times, you should notice the dogs ignoring each other. When that happens, you will want to reward them for the desired behavior. Again, use tasty treats like the hot dog slices. This teaches the dogs that you WILL not tolerate fighting in YOUR pack.
Learn body language so you can understand what they are saying to each other. These sites will help with that.
I hope I've given you great service. If so, I'd appreciate a 5 star rating. The rating should be based on my answer and not any site issue. Experts are not compensated until a rating is done. If you have any other questions at all, please reply here and I'll be happy to follow up with you even after you have rated. .