I am certainly no expert in these matters, but if I were your BIL and SIL I would be looking for a ped that had experience in dealing with children with delays. I'd hit the roof if a doctor told me my child wasn't bearing weight on their legs because they were unmotivated. My brother has lots of issues and the best thing my mom did was switch his pediatrician. He was much better connected to specialists than our previous ped. With respect to an official diagnosis, I do think that sometimes there are cases where there isn't a specific diagnosis that can be given. As an example, also with my brother, he's 28 and doesn't have an official diagnosis, after years of tests, including muscle biopsies when he was a baby and chromosomal testing as a young adult. He was also one of those "one in a million" cases, as all of the testing my mother had during pregnancy didn't point to any potential problems. Best of luck, I know these kinds of situations can be really tough on families.