7 weeks is a little young, was he a rescue? As for the training it may take a while as he is young, you could try with a toy as some dogs are more toy oriented, its easier with a solid treat but he maybe to young for that. while he is focused on the toy lift it over his head slowly, as he looks up and back his butt will lower to a sitting position. some dogs need a little extra encouragement to get the butt down for the first to times, by gently pushing on the butt whilst moving the treat over the head and a big celebration and reward. most puppies have a short attention span so you need to build up a strong bond and respect so he will listen and trust you more.