I'll just add this as an option, but I weigh my animals every week about 24 hours before feeding time. If nothing else I try to keep lots of data on my animals.

It's actually kind of neat to see how feeding your 101 gram snake a 13 gram rodent will only cause a 6 gram increase in weight a week later, even without defecating that week.

This kind of schedule would give your son a chance to see how feeding and defecation effect the weight of his new noodle, and it's a good excuse for weekly handling. Lol