Do you mean that they peck at him a bit and run over him? If he is getting pushed around a bit and you are worried about it try dividing your brooder in two with a bit of mesh or something and the heat lamp over the middle. Put the last hatched one and the next quietest/smallest one in together and the other two on the other side, just till the little one has had a chance to grow a bit for a couple of days. He still needs company and it's best that it's ducky company.