If your son is standing well and likes to stand (at 10 months, my DS just stood at the coffee table all day playing), then you might try helping him stand/squat on the toilet rather than sitting. Having the feet firmly planted helps give them leverage to poop - hence the advantage of a little potty that allows the feet to touch the ground, which I would also recommend if you haven't gotten one already. I help my DS stand with one foot firmly planted on either side of the toilet seat and me supporting him under the arms or around his chest. He faces the back of the toilet, so I keep a few books or toys on the toilet tank to hold his interest and keep him hanging out there long enough to finish. He often takes 10-15 minutes to finish pooping (it comes in several waves, and I wait a while to make sure he's actually done). During this time, he will generally alternate between standing and squatting several times, and this seems to help move things along. To minimize the splash from poop falling from so high up, I put some toilet paper in the toilet first. Also, you need to be on the lookout for him to start peeing too, and you may need to slightly adjust his position on the toilet to catch it all.
Mom to DS1 : 11/2004 and DS2 12/2008; happy to have discovered ECing, co-sleeping, and tandem nursing during our journey together