This was 1961, I doubt he was filling it out on a computer with a drop-down menu of items to select from. When filling out a paper form, nothing would stop him from writing Martian if he so felt.
The hospital form would be filled out by the hospital not Obamas father. What was sent to the US Health dept for their Natality report in 1961?
www.cdc.gov/nchs/data/vsus/vsus_1961_1.pdf