I am not sure if this is a foster that you are just taking care of while a home of his own is found, or if this is a dog named Foster.
If he is a foster dog that you don't plan on keeping, keep him separate from your male Lab and make sure that he is placed in a home where he can be the only male and the new owners know what same sex aggression is and how to manage it.
If he is your own dog and you plan on keeping him? Crate and rotate if you can't integrate him into your home successfully. Or rehome him if you can't crate and rotate.
Good luck. My own male GSD has other-dog issues and management is no fun.