The answer here is the same as I gave someone else this morning - pension records!
The best way to find out about his military career is to apply for the vet's pension records from the National Archives. These records should include a synopsis of the vet's military career, family info., and if you request the full file, affidavits from people who knew him giving first-person insight into his life. We explain how to apply for these records at http://www.spanamwar.com/geneaol2.htmhttp://www.spanamwar.com/geneaol2.htm
The trick here is that you should submit twice...once for each date of birth. You will only get a "hit" if they have records matching that data...so one should come back with a negative answer.
Incidentally, once you resolve the mystery, please consider writing a brief bio of MOH recipient Johnson for the Spanish American War Centennial Website. We would welcome the info.!