My little boy, who is almost three, has suddenly presented with a redness on his testicle, with a tightness of the skin. what could this be?

Consult with Dr.: This should not be diagnosed over internet. Some dr.S will comment on emailed photo + phone call others need to examine in person. Could be hydrocele, inguino-scrotal hernia, trauma, torsion of testis or appendix testis, epididymitis or epipipymoorchitis, cyst of epididymis & virtually never tumor. Pediatric urologist or surgeon can diagnose in seconds.

Many possibilities: I assume you mean a red scrotum. There are many possibilities including torsion of an appendix testis, true torsion of the spermatic cord (rare in this age group), infection, epididymitis, uti, hernia, hydrocele, etc. See your physician to figure this out, now if he is having pain!