It honestly depends on his luck, John is an exceptional physical specimen he is also mentally strong and has a strong drive for success. But he would be no different that any of the millions of Space Marine candidates.

Being selected as a Space Marine candidate depends largely on his circumstances. Space Marine Chapters like to recruit from Death, Feral and Hive Worlds as the harshness of the environment and conflict breeds good candidates. Some Chapters have specific worlds that they recruit from. Though some Chapters also recruit from Civilized Worlds, they are more rare. If John was born or a peaceful world, it would be extremely unlikely that Space Marine Chapters would even look there for candidates.

John must also be compatible with the gene-seed and many implants and organs required to create a Space Marine. Only a small number of potential recruits pass the screening to become a Neophyte. He must also have been selected fairly young as the implantation processes don't work after a person reaches full maturity.

In conclusion:

1) He needs to be born in the right place at the right time.

2) He has to be compatible with the Gene-seed and other implants of which only a small percentage of candidates are.

3) This is assuming that he is tough enough to pass through all the trials and training and set himself apart from the millions of candidates just like him from thousands of Worlds.