5 months is a little early to start feeding adult, We didnt change over until 10months. We didnt bother with junior food either
He needs the puppy food just now and all its added nutrients as he is still growing and developing very fast.
I am not an expert but I don't hear any of the more experienced members on here saying puppies should be thin - they all say they should be chunkier incase they get an upset tum...then they can cope better.
I am sure someone else will come along and help you soon.
Jane