Why would he necessarily think those who have tried to contact him are people? He could think only he and his tribe are people and the others are:
1. aliens
2. demons
3. spirits
4. some animal
5. lesser humans
6. and so on
Numbers 2,3 and 5 are not too far fetched. Heck even in the 20th and 21st century there are modern cultures that think this way. Racism is really all about #5. Various fundamentalist religions have a tendency to think in terms of demons and spirits.
It is a completely valid hypothesis that he believes he is the last man on earth or the last 'real man' or whatever.
When I was working in the Ecuadorian Amazon, the Waorani people, who had only recently been contacted, believed only other Waorani were human beings. This created several difficult situations because they saw nothing wrong with attacking and in some cases killing "non-humans" they came in contact with. My understanding is that many other native people in the Amazon used to share a similar view of their own unique human-ness
Why is that necessarily the case? Perhaps there are some aspects of this culture that make is psychologically easier for him. For example, maybe he believes that his people are with him just in another form.
I haven't read the novel or seen the comparatively recent film yet, but I saw both of the other film adaptations of the novel (The Last Man On Earth starring Vincent Price and The Omega Man starring Charlton Heston) and there may indeed be some parallels with how the eponymous man in either of those films views the strange, apparently subhuman creatures who roam the streets in the night. As other comments in this thread have noted, many indigenous people didn't recognize outsiders as quite human upon initial contact.
