First off, congrats on your new family member and thank you for rescuing from a shelter. You are correct that with him not eating right when bringing him home was a bit of adjustment period.
I must side with the others, SD is not a food you want to feed to your new pup. It likens to feeding your kids mac and cheese all the time. Alright periodically, but not on a permanent basis.
As a rule of thumb; you want a specified meat (turkey, beef, chicken, pork, lamb) or specified meat meal as the first ingredient. Corn is "alright" in a food so long as it's further down than the first five ingredients and is ground, not whole kernel and certainly not a gluten. I would aim for foods that are made and sourced in the US.
As you have a retriever mix, I would suggest checking locally about puppy kindergarten when he is old enough and follow up with obedience after he graduates