My ds1 couldn't roll over, get himself to sitting, pull to stand, cruise, crawl, walk or bottom shuffle until he started physio at 15 months. He was bottom shuffling by 17 months, crawling by 19 months and walking (after intensive practice!!!) at 23 months. The biggest problem was that he was always extremely tall and skinny for his age, so he looked really silly trying to push baby walkers along at 20 months! He is now 8 and runs about quite happily, but still has very lax joints and pronating ankles and hates being picked up, because it pulls his shoulders out of joint (he has a mild connective tissue disorder) and he will never be a great sportsman. He is now physically competent and active enough to be able to join in with everything other children of his age do at school, though, and to enjoy it - you wouldn't look at him and think he had any issues. He is also very academic, so gets his confidence boost that way.
The best thing the physio taught him at the beginning was how to roll over onto his front and then push back with his hands, letting his legs fold under him, and get into a sitting position. We also taught him how to crawl - he may not have been able to work it out by himself, but he learnt quickly if his body was put through the motions and he practised until he had built the strength to do it, himself. And he was taught how to pull to stand (lots of transferring weight from one bit of the body to another, which is tricky if you have low tone, weak muscles and loose joints). And he was taught how to crawl up stairs. And how to pedal a bike and climb playground equipment.... etc, etc. Basically, he was slow getting there, needed a lot of help to strengthen up his muscles (and had piedro boots to support his ankles for a few years and now has Sohli inserts in his shoes - they'll slot into any shoe - to stop his ankle pronation getting worse), but it was all worth the effort, because there's nothing he can't join in with, now. He turned out to be highly teachable!!! Playing the piano later on was also hugely helpful for his hand strength, which is obviously useful for extended pieces of writing at school.