Is it the 3rd eyelid (pink tissue in the corner of the eye) or the actual eyeball itself that is red?
I made a fool of myself one time with my vet because I thought my horse had an eye infection. I owned him for several years and one day noticed that his sclera (eyewhite) was pink. So I thought it was infected or had something in it. My great older gentleman vet, who has seen it all, told me his eye has always been like that, and it was something like the shape of his lens that caused it to look like that (I don't remember exactly how he termed it, but basically it was normal for my horse). And you know what, he was right. Even though I never noticed it before he eyewhite has been pink ever after and it causes him no problems, it is just normal for him. Really weird, because I am so detail oriented and I never noticed it.
However, I also had an older Paint gelding and the 3rd eyelids on him were always kind of inflamed and his eyes watered in the summer. I think it was skin cancer although he was around 30 at the time and I never had it actually diagnosed. The same horse had cancer on his penis.
Anyway, it could be something as simple as "normal for him" or an eye infection/something irritating the eye. Probably not cancer because he is still a youngster, but that can happen with older spotted horses.
Sorry I am not of more help, but I would be curious to know if it was the actual eyeball or the tissue in the corner (like the 3rd eyelid).
PS.................I just re-read your post and realized you said it was around the pupil. Sorry some of what I said is kind of irrelevant, lol!