Anytime a new foster arrives I treat them like a brand new, untrained puppy. They get limit freedom in the house (sometimes I tether even), frequent trips outside (supervised, reward when they pee, start training cue's). Crated when I cannot supervise. Freedom is gained with time, baby steps though like a puppy.
separately you can desensitize to the doggy door and he eventually hopefully learns to follow the other dogs in an out.