"Loose stools" as side effect from medication

[GerbilsAteMyCat]

Hi,
DS1(8) has been on antibiotics for a severe infection for several weeks. One side effect is that he has accidents, basically heavy, fast bowel movements that he can't control. I have thrown out a pair of pants nearly every day this week because they are beyond saving.
The hospital doesn't want to change the medication because we're nearly at the end, which I understand.
Is there anything we can do to help stop the flow? We have stopped soft fruit, increased bread.
What can we do to block him up (until late next week when they stop the medication).

Give him lots of rice and yogurt.

Maybe consider grown up nappies at least while he's at home?

Thanks!
He is eating lots of yoghurt to help with the anti biotics. I'll look into rice. Would rice cakes do the same thing?
I think he would be mortified with adult nappies.