Well, you have to remember, he will not know why his body is doing this as he does not recognize the signs fully. He is still learning to know when he has to go. I recommend putting him on the potty before you put him in the tub. I would also push the tub time up a bit as his schedule allows to see if he will go on the potty.
The finger may just be him exploring his body? Check with the ped on that one.
As for constipation: I do not understand why anyone would recommend fiber to a child. Fiber 'fills' up the stomach also meaning he will feel full and not eat as much for you. This could also be why he is pooping at a different schedule and it is hurting his bottom (and why he is putting his finger there - thought just came to me on that).
When a child is constipated, diet and water are your biggest things to fix. Usually it is not enough water intake (mostly milk) and they need more fresh fruits/ veggies and less meat. You can also get around ANY Over the Counter Product for Anyone by rubbing the belly a specific way and in doing so; this helps Anyone eliminate with ease. I rub my son's tummy all the time and now I had to teach him how to do this as he is in preschool (I told him it would be rather hard for Mommy to go to school and help him there. Now he's being goofy by adding the tickle at the end as I did! LOL~)
Good Luck!