That's why they make stretch pants! And tights! My boy had (has) a miniscule tush. He wore training pants long after his need for them was gone, just so his pants would stay up. We still have trouble with pants staying up on him (girls stretch pants are baggy, 4t is sometimes big in the waist and short in the legs - he'll be 7 in 2 weeks) This kid was still wearing 0-3 month pants as shorts until he was 4, and he didn't grow out of them, they got too stained to wear. If he could sit on a potty by 8 weeks, I'm guessing you are having some other problem with the potty than the size of your guy's heiny.
With that in mind, try, try again! Little guys don't perch upon the potty like a chair, it's more straddled like a saddle. When young walkers approach a potty, it's often from behind or stepping one leg across. If you sit straight back you get the uncomfortable feeling of falling, rounded back and you wind up sitting farther forward on the potty than you hoped. Go ahead and try it, I'll wait. ... Ok, so if you put one foot on either side of the potty and then squat down, you're going to be in a much more comfortable position. More upright, tailbone touching the back guard. This is the position you want your son in. He won't fall in because his weight is supported by his thighs, almost out to his knees, rather than on the far edges of his cheeks. Much in the same way tiny kids use a toilet without falling in. Practice, balance and weight distribution.
So how do you get your son to straddle the potty? Planting his feet and then getting him to bend his knees could proove tricky. You can try it from above. Hold him in the classic position above the potty, with his sacrum touching the back plastic. Then seperate his thighs as you lower his feet toward the floor on opposite sides of the pot. Support his upper body, move in front of him so you can see his face. Smile!
Good luck! Let me know if you get it to work.
Seraf
