Smelly, gritty poo is definitely a sign of impaction, I would disimpact. New poo often sneaks around old poo, hence the normal looking stools. Is he currently taking anything to help him go?
If you are going to use suppositories definitely seek medical advice. the only issue with them if the blockage is higher up in the bowel rather than lower down they will have zero affect and it will be pointless using them. Some people find making up the days movicol with the correct amount of water and leaving it in the fridge and then adding it to drinks throughout the day, this is a really good way to get the movciol in and to ensure that they are drinking enough fluids.
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