Personally I feel that the other boy is going through the "gay phase" seeing as he's 13 (personally I don't believe that sexuality can be set-in-stone till someone is bout 17)

All relevant research and the bulk of anecdotal evidence has concluded that sexuality is set very early in life, long before puberty. If it makes you feel better to believe otherwise, that's perfectly fine as long as you are aware your belief flies in the face of all known research on the topic.