Your elder son is also on the cusp of being a full-fledged teen and may already be carving out his own separate adolescent identity from his "little brother". You can't really stop your elder from putting down your younger boy out side your presence, however you can certainly forbid any such hurtful talk in your presence. I suggest doing an occasional one-on-one check with each son to talk about their relationship. Empathize with each one about the normal difficulties of having an older/younger brother at this age. Tell your older son you expect him to demonstrate his maturity and "elder statesman" qualities by treating his brother how he would wish to be treated; praise his efforts to do so. Siblings will "rank" on each other, after all, but if a family member is truly causing another pain then that harmful behavior needs to stop.
Toddler and Teenager Expert Advice from Carleton Kendrick, Ed.M., LCSW