Sounds like Rex may have made a breakthrough I understand completely how encouraged you were by his positive encounter! Aly's given you a lot of great advice I can only re-enforce that lots and lots of continued socialization experiences under control circumstances should help greatly in getting Rex to accept and enjoy other dogs! Do you/have you attended obedience classes with Rex? That's a great place for lots of positive and controlled interaction! Keep up the good work Rex! You made your mommy so proud