Younger dog wont leave older dog alone

Question
I have two small male dogs, both neutered. Kipper is 5 (bichon shih tzu mix) and I've had him from a pup. Mojo is about 1 year(cavalier shih tzu miz) and I adopted him 5 months ago from our shelter. The pair do pretty well together, no food issues or jealousy that I can see. The problem is that Mojo will "go after" Kipper and start biting his neck, feet and tail. Kipper is passive and just cries and tries to get away, sometimes he'll get really frustrated and it is clear he is trying to tell Mojo to "leave me alone" but Mojo persists. Either Kipper is too passive or Mojo is too determined. I think Mojo is doing this to initiate play but I don't understand why he doesn't stop when Kipper yelps and tries to get away. It upsets me as I wanted Mojo and Kipper to be able to be together while I away and instead Mojo has to be separated.  Do you have any suggestions as to how I can fix this?  Or any suggestions as  to what Mojo is  trying to  accomplish?

Answer
I'm unable to actually SEE this interaction and I very much doubt you are reading body signals correctly (because most people don't know HOW, we can't ALL be dog behavior experts).  Mojo may never have learned how to "read" dog body communication, he may be the product of some backyard breeder who knew nothing (and cared less) about how to manage neonates, he may have been separated from his litter mates at so young an age that he has no idea how to interact properly.  I very much doubt he's attempting to "initiate play", it's more likely he's attempting to obtain rank over Kipper.  At age one, he's obtained the first plateau of maturity.  Kipper is giving clear signals of pain but is still apparently attempting to put Mojo in his place (per your report) with no success.  These dogs HAVE to be separated when you're not at home, regardless of the reason you obtained the second dog.  Same sex is never a great idea, adopting an already five month old pup from an uncertain background is difficult without serious temperament evaluation and, even then, requires diligent supervision with the other dog(s) in the household.

Mojo needs to be on a house tab: this is a lightweight (handle cut off) leash.  YOU must intervene when Mojo interacts in an appropriate manner with Kipper: pick up the leash, go quickly to another room, put him in there, close the door between you, count to ten, let him out.  Observe his behavior: if he begins anew, repeat.  This isolation, while lasting a mere ten seconds, will have (after repetition) an immediate correlation to Mojo between the behavior that caused it (you must intervene as he precipitates the obnoxious display) and his isolation.  Meanwhile: put Mojo on Nothing In Life Is Free (NILIF).  The regimen requires he respond to a "sit" command (use a new word), training for which is found here:
http://home.gci.net/~divs/behavior/bemod_relax.html

The NILIF program was designed by a British Veterinarian and psychologically places the dog into a lower "rank" in the household, while placing the human in a higher "rank".  You can read about it here:
http://www.vin.com/VINDBPub/SearchPB/Proceedings/PR05000/PR00470.htm

NILIF should be maintained for Mojo for about a month, then SLOWLY removed while observing any effect of the removal.  Should he begin again, put him back on the full NILIF for another month.  Meanwhile, learn about dog rivalry in the same household:
http://www.azhumane.org/PDFs/behavior/dogs/caninerivalry.pdf

AND how we, out of love and kindness (usually), create problems in our dog companions:
http://www.dogbreedinfo.com/articles/humandog.htm

You are clearly not in control here and Mojo knows it (so does Kipper).  By using NILIF, you will be giving Mojo a clear signal that you ARE in control, but you have to follow up with some real intervention and that will include positive reinforcement training.  I suggest you go to DogStarDaily.com for information regarding how to teach BOTH dogs "commands" that will put you squarely in control while THEY are VOLUNTARILY offering the behaviors.  This is Dr. Ian Dunbar's website and there's an enormous amount of information there regarding training and behavior modification.  Regardless of why Mojo is offering this behavior toward Kipper (and it's clearly not his "fault"), you do not have to tolerate it.  Using dog psychology, you can help Mojo to understand that certain behaviors will result in unwanted results, while others will result in reward.