Depends on what the operation was for. I havent heard of anyone getting sick from taking a normal dose of a laxative, if you take too much you can get very ill and dehydrated. It also depends on what medication if any he is on too, they might not mix well with Senokot. You could try a milder laxative life syrup of figs, or dulcolax. Failing that, good old plenty of water and fruit and fibre should do the trick in time, but try not to panic if he hasnt passed anything, unless he starts to feel bloated and generally fatiged due to not having a movement.