Puppy development questions

Ok so I just wanted some opinions. Dexter's obedience trainer made the comment that he was overweight. She use to show dobermans in the ring in AKC so dobes are her breed. I just wanted you guys' opinions. I always thought puppies were usually on the heavier side and really didn't want to put him on a 'diet' at this age (he is 5 months). Also, she said he was down on his pasterns a little and to start giving him vitamin C....we started that a few weeks ago, very slowly. I noticed he has been tucking his butt a little more so his top line is rounded instead of straight like it use to be. Are the two related? Will it probably straighten itself out when he matures? He just went through a huge growth spurt and thankfully has slowed down. Here are a few pics I took while playing fetch and working on commands...unfortunately I can't get a nice stacked pic of him by myself and the hubby is absolutely dreadful at taking pictures. He is an awesome little man and I just want to make sure I am doing the best for him.