A lot going on in his little life he could bve anxious and if he hears grown ups concerned for the son in and out of Hospital thats enough,hes worried for his brother I'd say, how does your Mom handle him ,does she reassure him his brother will be okay, does she do plenty of things with him, reading ,Games ,going out, Treats,Take his mind off the problem, and relax on the potty training ,Maybe hes not with you a long enough time each day and its a bit of Seperation anxiety, give him more time before he goes to bed, dont be rigid on the time he has to go to bed, read him happy stories and have some quiet time, get Dad involved , ask Dad to take him out Play ball, do guy things.I am sure you do pay him attention but is it the right kind .