S.B.
To solve the immediate problem of him digging in his pants, try putting a pair of tights (like girls cable tights, do you know what I mean?) underneath the backwards jammies. That will stop the going up the leg issue.
As far as the overall issue, I would try a couple of things:
When he does it, don't react AT ALL.
Make sure that he has plenty of play dough to play with during the day to curb the stimulation he may be getting from the texture.
Try feeding him lunch and dinner 1 hour earlier than normal so he has time to get his business done before nap/bedtime
You don't mention if he is still on the bottle, but, if he is, eliminate it. I don't know why it seems that sometimes those two are connected.
Good luck :)