My advise is to tell him about what you have learned from him and wish him luck in his future. If it is a close enough relationship ask if you can keep in touch with him and if he agrees, then take the method he provides to you. And then send him an additional thank you for being a role model to you and tell him something about yourself he may be interested in, in the context of your contact with him, and your plans for the future which he may be interested in. Sign off wishing best regards and hoping he would stay in touch.
