Reading what you wrote was like reading about my own son. Patrick is 2 1/2 and was recently diagnosed with NF 1. He experiences the same leg pain you described. He also has trouble with the gross motor skills- climbing stairs, pedaling a bike, jumpping, etc. He only complains about his left leg and it is completely sporadic. Sometimes it happens during the night, as you said about your son, sometimes it's during the day, sometimes it lasts for minutes, sometimes for hours. There are times Patrick can't walk at all and will actually get around to play by crawling. It seems to have gotten worse and more often, he seems to walk with a limp quite often, even when he's not complaining. Before we found out about his diagnosis, Patrick saw orthopedics and rheumatology, neither could come up with a reasonable answer for the pain or what was causing it. Since the diagnosis, we've been back to the neurologist about the leg pain and she sent Patrick for the same MRI your son had on the lumbar and sacral spine. We will get our results tomorrow. I'm not sure what segmental NF is, but from what I understand, the neurologist is concerned that if there are tumors growing on or around(?) Patrick's spine, that they could be compressing a nerve, causing the pain in his leg.
I am intersted to find out what happens with your little guy, as I am afraid to find out about my own tomorrow. This diagnosis has turned my world upside down. Patrick hasn't even been seen by the Neurofibromatosis specialist yet, so I'm not ready to find out about any complications that are being caused by this NF. Please keep me updated. I feel like we are in very similar situations and I will be wishing and praying for the best for both of our boys. I hope to hear back from you with good news.