You need a command to go along with “good boy”.
Every time your dog goes to the toilet, say your command, even if they do have an accident on the rug. You need to say it as they are doing it, not after. With consistency they will start to realise what the command means.
Romanian rescue dogs have been used to toileting outside for their entire life so they are likely to feel more comfortable in the outdoor space, what they are not used to is being shut in a house, unable to get outside if they need to go to the toilet.
It is likely they will have a few accidents in the home whilst they are settling in. Due to being neutered later in life, some males can mark their territories which could lead them to urinate in the home space.
It is important to go a bit 'overboard' in the first week with toilet training. Get them outside first thing in the morning, frequently throughout the day, after meals, naps and before bed.
If your dog is refusing to go we suggest putting a coat on and waiting them out. They will soon give in. At this point they need to be praised and then allowed back inside.
Be patient with your dog.