Is my dog not getting enough food?

i can feel his ribs slightly, and when he lays on his side i can feel the bottom part of his ribcage. i feed him but, my other 2 dogs (small terriors). the dog with the problem is a shelton sheppeard. hes a medium sized dog i guess. hes fed but the 2 fat littel small dogs eat his food then run over and eat there own. i hope they both die. how can i get my dog to eat better, hes afraid to eat now most of the time because the other dogs bite him when he tries to. i cant get my mom to discipline her dogs and let my dog eat, she loves them to much. im getting close to beating them half to death when she isnt home. is it normal to feel his ribs a littel, i tried grabing onto his fat but he has nothing to pull up ( i try to squeeze some skin between my fingers and see how much slack it has but i can only get it up like a centimeter ... u know what i mean).