9 months is a little early to say autism or PDD.
Not every developmental delay is autism nor PDD. Not every isolated developmental milestone missed is even a developmental delay.
Some physical milestones like sitting, crawling, standing walking espeecially might be signs of something else.
It's okay to watch the milestones over time and note them and talk to a Ped.
Kinda like a tornado watch. Maybe some conditions in the sky look like a tornado could form, but you don't need to run for cover yet. I don't have to live my life in the basement ; I'll just put my golf clubs away and miss my tee time. Then it gives you a heads-up when you hear the sirens.
It's good that you are paying attention, tho. Some moms don't look, don't care, or don't see anything when they look for years.
My son was delayed in all the gross motor skills. But then again fine motor skills he was on time. He was a big boy for his age. He never officially crawled. He did this army man scoot. He sat up late also. I knew he could do it. I'd seen him do it & then he just kinda refused to do it again. Until later. He didn't walk until he was 18 1/2 mos old. but once he began walking that was it. He was off & running. so he was behind a bit but caught up in the end. I've decided that he's done everything on his own timetable. but he's very intelligent, affectionate, loves to play & as far as I'm concerned, completely normal. He does the flapping w/his hands as well. when he's excited. I've also read this to be a sign of Autism. But if everything else is normal then I wouldn't worry too much. Perhaps think about some PT. we did that w/our son when he wasn't walking yet. He'd been cruising for 8 mos w/o walking solo. & you know what? he started walking literally the weekend before he started therapy. He did that over & over to us. right when we'd think something was "wrong" he'd do what we were waiting for. so I say, he has his own timetable. but certainly never hurts to check w/his pediatrician & consider some kind of pt to help him catch up. best of luck!