While in the Savage Land, Piotr aka Colossus had an encounter with two native women, Nereel and Shakani, who wished to make love with him after Colossus saved them from a thunder lizard. Nereel wished to bring new blood into their tribe. Piotr made love for the first time. The fruit of their union was a son, Peter, born long after Piotr, the elder, left the Savage Land. Colossus is unaware that Peter is his son, despite having met him. (this all happened in various issues of X-Men some decades ago real time, I'll say 13 years ago using Marvel's sliding timeline?)
So Peter was left to grow up in the Savage Land raised by his mother and her tribe the Fall People.
During their Secret Invasion Earth, the Skrulls noted that Vibranium (and subsequently the weapons created from the metal) could hinder their plans. This led to the mining of Savage Land Vibranium under the guise of a rogue S.H.I.E.L.D. faction. They attacked many local tribes and took many "natives" as slaves (also actually happened in comics during Secret Invasion).
Peter (Jr.) was one of these slaves that they made mine the Antartic Vibranium known as "anti-metal" and managed to free himself during the altercation between the Mighty Avengers, New Avengers and Skrull Avengers. (the altercation is also canon)
Thirteen year old Peter found his prolonged exposure to anti-metal had triggered his latent mutant ability. Like his father he could turn his body into organic metal....Anti-Metal! In his metal form he had the properties of Antarctic vibranium.
Weeks later Cyclops and Emma Frost have taken a holiday to the Savage Land, but are called to San Francisco by Archangel because a hippie 'goddess' (actually Martinique Jason) has placed the city, as well as Hepzibah, Warpath, Iceman and Archangel, under her control and in a 60s style illusion. (canon from X-Men Divided We Stand). Using his hunting skills Peter managed to stowaway in their unnoticed (though Emma may have actually detected him but let him stowaway anyway, probably just to see the look on Colossus's face when he met the boy.)
While in Frisco Peter made his way into the city to unbeknownst to the X-Men (save perhaps Frost) to acclimate himself to this strange and wondrous foreign land.
This should bring young Peter directly up to Marvels current timeline. I'll leave it up to Holyman about the rest.
Also if we need to alter anything history wise to fit the plot I am more than willing.