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Keeping him on towels (preferably white and without pattern) is a good thing to do.
You can see if he is pooing alright, if there is any stuff oozing out of the wound, also there is no chance of pet bedding to get stuck on the wound. Towels are a def. a good idea. I would put newspaper and woodshavings underneath the towels though, simply because the towels alone dont soak up much urine and you will have LOADS of washing up to do if you use them alone.
Also make sure he isnt nibbling on his stitches.
When I had my Marvin neutered everything went fine, he left the stitches alone, and had a good appetite and regular bowel movements, after a week I went to the vets again for a checkup, and everything was fine.
I mean obviously there is always a risk involved with an op, and afterwards wounds can get infected... but in general castrations are a routine thing, and there shouldnt be many problems.