My son has a money collection, in the money collection are Japanese pesos...WW2 stuff. Whats the story behind Japanese pesos?

It is money issued by the Japanese for use in the Philippines.
The Japanese invaded Manila on 2 January 1942, and in the process captured more than $20.5 Million in U.S. and local cash and an unknown amount of foreign currency and bullion The Japanese used this hard currency abroad to purchase raw materials, rice and weapons to fuel and feed its war machine. In its place, the Japanese issued several series of fiat currency. The Japanese military confiscated all hard currency, both on a federal and individual level, replacing it with locally printed notes bearing a proclamation of military issue. All notes bear the name of the issuer, “The Japanese Government” while some notes proclaim the “promises to pay the bearer on demand”. Called “Mickey Mouse Money” by local Filipinos, it was valueless after the overthrow of the Japanese, and tons of it were burned. Japanese troops were ordered to destroy bank records and any remaining currency prior to capitulation.

By the end of World War II, the war ended and the currency circulated in the bearing of the Japanese name immediately become useless papers. Japanese Invasion Money as it was called was issued in a number of countries such as the Philippines, Burma (now Myanmar), Malaya, North Borneo and Sarawak (now Malaysia), Singapore, Brunei, the Dutch East Indies (now Indonesia) and some areas of Oceania (New Guinea and the Solomon and Gilbert islands).were deemed useless.

...

They were issued as currency during the Japanese occupation of the Philippines in world war 2.

most countries that occupied countries issued script or Military Money an occupied country has no currency support in Germany post WW2 we had British BAFS's

British Armed Forces i kept a few i have a 3 penny BAF and a shilling BAF the USA had script in Korea

and the Germans issued Reichskreditkassen which was their occupation script

The Philippines probably still used pesos, and Japan occupied the Philippines for 4 years.

It's not surprising at all that Wiki-wacky "didn't have much info." Wiki-wacky is worthless-worthless.