Brief Answer:
Normal in most of the babies

Detailed Answer:
Hello

Thanks for asking on HCM

Child of this age are very exploratory in nature. They start their exploration from their body first. It is normal to explore different parts of the body and also feel the sensation by touching them. Parents get more worried when child touch the undesired part like genitalia or anus.
Sometimes it is related to toilet training also as this the age when toilet training is at its peak.
I suggest you not to overreact rather try to make him understand that the anal area contain germs and putting finger is unhygienic. Wash his hand properly with soap and clean the undernails with nail brush to avoid spread of germs.
Most of the time this habit goes as the child grows.
Still I would request you to get him examined by a pediatrician to rule out worms or any anal irritation.

I hope this helps.
Please let me know if you have any more doubt.
Regards

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