Some dogs go through a teenager stage right around 8-18 months, he may just be rebeling right now & may get over it.
Also you said a new rug, maybe introdice new items to him slowly so he does not feel threatened or mark his territory. Also just ignore him when he goes potty inside & bring him outside right away. EVERYTIME you bring him out to potty go with him so you can praise him when he goes (just like re-potty-training him). You say he is scared of the outside so if you go out with him that will help, and also sometimes just out w/ him to play so he knows there is nothing frightening out there.
And keep a good eye on your neighbor, if he pushed your brother off his bike & your deg watches him there may be a good chance he is scaring your dog? And inroll in obedience classes as it will help train your dog in numerous ways, he will be smarter, he will bond more, he will have more fun & he will be better socialized. Try to bring him places w/ you to help socialize him & to show him that he goes outside to have fun. But the biggest part again is to PRAISE hwen he pottys properly.

Is there anything differnt that may have changed? His schedule or food or anything? That may trigger this behavior.
good luck