Why Is My Son S Eyeball Red?

Hello, my son is 3,5 y.o., yesterday I noticed that half of he s eyeball was red ish, and there was almost like a spot (it s definately risen) just outside he s pupil. it also seems to make the ring of the pupil blurry. he s ok himself, doesn t seem to itch ( he says it doesn t itch anyway). I know it s impossible to say what that is just from this description but any ideas what can it be? Thank you