Sharing one hotel room in Japan

Is it weird in Japan for mom and son to share one hotel room?

No, not at all. Why would it be?

If you're trying to save money, in my experience the price of a double room is about the price of two single rooms in Japan and, especially in the big cities in budget hotels, a double room most of the time has just a small sized double bed, not the two separate single beds you often get in other countries.

in my experience the price of a double room is about the price of two single rooms in Japan

In my experience, it is not.

and, especially in the big cities in budget hotels, a double room most of the time has just a small sized double bed, not the two separate single beds you often get in other countries.

A double room means a room with a standard-sized double bed; a room with a small double bed is a semi-double room, and a room with separate single beds is a twin room.

If staying at a business hotel - just book a twin room. If your staying at a Minshuku or Ryokan, staying with friends and family in the same room is considered normal.

In general, twin rooms are often 30-50% more expensive than single rooms at many business hotels. Hotel chains such as Toyoko Inn's allow you to book and see the general price of rooms if this is a consideration.

The original poster who stated that single rooms are the same price normally may have been talking about traditional accommodation such as Ryokan's or Minshiku's which tend to give no or only a small discount to sharing a room with another person. This really depends on the location to book a room at.

Like many things in Japan, unless you are being a huge imposition or are making a large commotion, most people will not even look or think twice about a mother and son (even adult son) sharing a room.

If you give us a budget, may be we can suggest a hotel, but you can consider a ryokan, hostel or some cheap capsules.

Why would it be odd ?

No.

That doesnt seem that out of the blue