I am typing this as I sit here and wait for DS4's PT to get here. He does PT and OT once a week.
DS4 did the commando crawl, sat up late, could not hold his head up when he should, and could not stand and put pressure on his legs at 12 months. At that point, the ped sent us for a PT evaluation. My son was diagnosised with hypotonia(low musle tone) in his trunk and hyperflexibility in his joints.
Hyperflexibility in your muscles is good, think gymanast, in your joints, it is bad and can cause knee and hip problems. He could put his leg in places no man should be able to.
Hypotonia in his trunk is bad because you have no core strength, therefore the extremities suffer as well. If you are familair with Pilates this is the whole concept, strenghten the core and the rest of the body will fall into place. He "w" sat as well, and this was because his core was not strong and sitting like this put unnecessary prssure on your hips.
Also if your core is not strong this can affect fine motor skills as well which is why DS4 gets OT as well, he also has some sensory issues so they work with that too. When he was first learning to walk, he would "slap" his foot harder than you or I would to have our brain send the signal to take the next step, he would slap harder to get that feedback to his brain.
They did tell me he could outgrow this and walk fine but that is was better to get on this issue early.
Ok the PT just left and I told her about your concerns, and said yes press for an evaluation, it is the best way to know for sure.
My older son exhibited some odd walking when he was 4 and he was tested, he scored 2 pts too high to need formal therapy and told me to keep him involved in sports, doesnt matter what but to keep him active.
I too have been the victim of some nasty comments ,even her on the DIS, by someone who was suppose to be a nurse, and didnt understand the difference between fat and low tone

scary isnt it.
Anyway now most people dont know he has any problems, unless they get behind him on the steps but even that has seen much improvement and that his run is not as graceful as some other kids, and endurance is still an issue, but we will get there!