Hi, my son has a similar friend and wasn't sticking up for himself either. The little boy will also pull him around by the shirt to wherever, hit my son for no reason, be bossy etc... My son had his own methods of dealing with him like running away from the kid. I would see my son running (fast) with a toy and this other boy chasing him. Or my son would hit the boy back or just scream stop at him.
I was more confused by my son's friend's parents not doing much to interject and correct their child. So, I started to step in.
I now will step in nicely and stop the situation. If the friend wants to play tag and is demanding my son to play. I say (son's friend) John ask steven to play tag -- I have him say "do you want to play tag?" I then say to my son "Steven do you want to play? it's ok to say no!" Steven will then say "no". I will tell John he doesn't want to play and ask if anyone can come up with another idea.
Or if John is taking a toy from Steven -- I make them stop and make John ask for it and have Steven respond then deal with it from there.
If John hits Steven -- I firmly tell John we don't hit and I have my son tell him "Don't hit me! I don't like it!" and if John continues I say something to his parents like "We don't play with kids that hit." and leave.
I started doing this in front of the parent -- I told the parent I am not correcting your child -- I am helping my child to learn how to deal and cope with other children in a positive manner. This is not about your child but, about mine.
It has done a couple of things -- John has gotten better with social manners (my son) and it has helped my son practice voicing his feelings and wants/needs from other children. John's parent have even started working with the boys.
The other thing I have done is told my child that how John is acting and treating him is not a reflection on you or who you are -- it's a reflexion on John. I tell Steven that if he doesn't like how someone is treating him -- it's ok! not to want to play with that person or even like that person, but, he still has to be nice to John or whoever.
I hope this helps.