UA I used to get comments from people asking if #1 had been unwell, if he had enough feeds, etc...he was a long and skinny one too. The only saving thing for me was that in my head I KNEW that he was eating enough, his output was fine, he was happy, chirpy, not lethargic. He just didn't look like all those chubby delights. You talk about leg rolls - that sounds good, #1 never had those. Stringbean to the max.

If you think there's a problem, then get him checked out, but if the only issue is that he looks more slender than other babies, I really wouldn't worry. I think that people forget about genetics sometimes.
And, funnily, I didn't change much with my parenting of #2 and he was as roly poly as they come. My stringbean and butterball.