One-legged crawling?

[stopgap]

My DS is 11-months-old and has "crawled" since 9.5 months with his left leg in front (at a right angle) and his right leg pushing from behind. He started pulling to stand at 10.5 months and it appears that his left leg is weaker (it kind of buckles at the knee when he's up and sometimes he rolls over the left foot, though he does maintain balance pretty well).

Has anyone been through something similar? Our paediatrician has just referred us to Early Intervention for gross motor evaluation and I'm trying to get a handle on what this sort of thing involves.

DD used to do this for a while, and then she moved onto bum shuffling.
She has low muscle tone. Don't really know if the two are connected.
She also goes over her ankles but this is because the arch in her foot is developed properly yet (flat footed) physio said it will develop in time.
At the moment she has shoes from orthotics with speacial insoles in them to support her ankles and feet when walking.