Sussan451,
When I've done 2 enemas per day I will do one in the morning after my BM"s and one in the evening before bed. Its great that your son can retain the enema during the day, if he ever has troubles retaining the day one you should ask your docctor for foam enemas(cortifoam). I use this one during the day because its easy to retain and its a foam so it absorbs in quickly and it sits in the lower area of the colon.
Is your son taking any probiotics daily? Very beneficial to replace the good bacteria in the colon.