15 weeks and you are gettng worried already????? He is still just a puppy and wants to play puppy games. If he will retrieve the ball then keep that up at least that works. As far as him being tired try a higher protien food and also canned food to go along with it to give him more energy. If you are really worried about him not retreving the bumper use a smaller bumper or different colored. Dont worry about putting fethers or scent on the bumper for now because he wont be retreving bumpers when you are duck hunting any way. I would wait on fethers and scents until this sceason when you shoot a duck bring it home and try to have him retreve that if he does not at first no big deal put it in the freezer and use it for another day. I do have to agree with the other person who talked about obedience and socializing. I have had dogs that dont retreve until they are a year old. For example my wifes dog. If all else fails and you like the dog and the temperment get him force broke to retreve.