Firstly, Ichitaro is a very common boy’s name.
The family name Nishizawa is also a common family name.
It is different from the family name Nishikawa/Nishigawa, which is also common.
On Ancestry:
UK, WWII Alien Internees, 1939-1945
Ichitaro Nishizawa
Japanese
Birth, 18 May 1898 (1939 Register shows 27 May 1898)
Resident
Interned 8.12.41
Monmouthshire Constabulary
Release refused 26.9.44
R.A. 24.8.45 (S.C.)
Subject to 6A & 9A
It is just a brief hand-written card, but there may be further information somewhere.
It also says H.O. No. N 7625 (a Home Office No.?)
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Reply #2 mentions a 9-year-old boy at the St. Albans Vermont crossing who arrived in 1907. This is a record of a crossing from Canada into the US.
Family Search (free of charge to view, but must register) has the actual image for this record. Actually, there are 2 images, but they seem to refer to the same boy. Probably there was some confusion about how to write his name, and two cards were completed under different names, presumably by mistake.
1) Image 8108 of 8204
https://www.familysearch.org/ark:/61903/3:1:3QS7-99DQ-L9C7-8?i=8107&cc=2185163Manifest. Port of Vancouver B.C.
Nishigawa Ichitaro
Nationality, Japanese
Place of birth, Japan
Age, 9 years
Occupation, Actor
Read or write? No.
Last permanent residence, Niigata Ken (Niigata Prefecture)
Passage paid by Manager
Ever in the US? No
Destination, Fairbury Neb (Nebraska)
Date of Landing, 3/24/07 Monteagle (ship's name)
2) Image 8106 of 8204
https://www.familysearch.org/ark:/61903/3:1:3QS7-99DQ-L9C6-X?i=8105&cc=2185163Manifest. Port of Vancouver
3rd class Monteagle (ship’s name) (from) Yokohama
Date 3/24/07
Nishegawa Ichetara
Nationality, Japanese
Place of birth, Niigata Ken (Niigata Prefecture), Japan
Age, 9 years
Occupation, Actor
Read or write? No
Last permanent residence, Niigata Ken (Niigata Prefecture) Japan
Ever in the US? No
Destination, Fairbury Neb (Nebraska) c/o Campbell Bro. Circus
Purpose in coming, Perm (permanent?)
The cards give the name of the prefecture, but they do not specify the name of the city. There is a city named Niigata, in Niigata Prefecture.
Reply #2 also mentions some other names of people who accompanied him. If they are connected, you should be able to find their cards too. That may offer the name of the Manager, or some info on the other names, or the name of a city.
In Japan, there is no register of individual births, marriages & deaths. There is nothing like free BMD or the General Register Office. There is no hobby of genealogy. There is a Family Register (Koseki) for each family which is kept in the local Ward Office of a city (something like the local council offices) where a family lives. Whereas each child in the UK has their own birth certificate, with no reference to their siblings, the Family Register will list all the siblings. Every family/person must be registered in their local Ward Office. However, it is illegal for anyone to see another person’s Family Register. Only a family member can have access to it, or a lawyer. It is considered private. So even if you had an address in Japan, it would not be possible to check on this boy’s parents, without a lawyer's help. The only hope really would be if the boy/man had provided further information about his parents to the authorities in the UK or US, for some reason or another, or if a parent had accompanied him on the ship.
You can google the circus, it seems to be famous.
Anyway, I hope that this provides some answers to your mystery.
Family Search also has free access to the US census images (e.g. 1910 & 1920). Perhaps you may find him there.
Canada also provides free access to their census images online.
UKgirl