Something most likely has happened to this poor guy. Aggression issues aren’t something that will go away in the near future, but lots of hard work, patience, and positive training will eventually (hopefully) lessen that problems.
First, I would always have treats available when you’re working with him. When getting ready to put a leash or collar on him, have treats, and counter condition that act to be a good thing. Make sure you approach him from the side with your body turned so you aren’t coming directly at him. Squat down, hand to the side, ive him a treat. Get another one ready for while you are leashing him up, and give another one, with praise for putting it on well. Do this slowly, and if he reacts, back away. Wait for him to calm down, and try to repeat the process. Don’t let him greet anyone on the street, if they get offended, explain he is a foster with some aggression issues you are working on. Anyone that you may have in his space, make sure they are using proper dog language to greet him, and no one is rushing the dog.
Resource guarding can be tricky- toys shouldn’t be left out for the dogs unless you are there monitoring, and if things turn from friendly rough play to aggressive guarding, the toys and treats go away. Reward good play and sharing with treats
Good luck
