Will he STOP on the whistle when walking at heel? if not start from there, no point asking a dog to STOP 30 yards away when he wont STOP at your feet!!
It sounds like in the past you have made a habit of recalling after getting him to STOP, it sounds like he has associated the STOP whistle with the RECALL at some point. If you do RECALL after a STOP command dont let him get all the way back to you, STOP him about 10 yards away from you and then you go to him and plenty of praise.