Canine Aggression

[gwscwm]

My 4 month old terrier cross is fantastic with dogs of all ages and sizes - plays when invited to and backs of when another dog signals it doesn't want to play. However he is being dominated by another terrier cross puppy who is 2 months older than him. This dog lies in wait for him and then launches on top of him. If my dog lies down the other will lie down on top of him; if my dog tries to get out from under him then the other will attack him - he has bitten his ear and punctured his eyelid. His owner thinks that her dog is just playing. I am wary of trying to discipline another persons dog yet do not want mine to end up nervous - at the moment he still wants to play. Unfortunately avoiding this dog is not an option as we walk to the same school and parks - also I would rather find a way to solve the problem than avoid it.
Any advice will be appreciated!