It is unclear from your description if your brother is having any associated complaints such as itchiness, watering, or other associated symptoms other than the redness in his eyes. There are many things that can cause red eyes, and the entire differential is beyond the scope of this discussion. In terms of some of the major possibilities, as you mentioned, allergies, colds, as well as infections and dry eyes can be very common causes for redness of the eyes. Abuse of certain drugs such as marijuana can also lead to chronically red eyes and if there is a concern for this, there are blood and urine tests that can screen for drug use. It would be advisable that your brother be evaluated by an ophthalmologist
or primary care doctor
for further work up of this condition. They will be best able to evaluate his medical history and presenting symptoms, and decide upon the appropriate work up if deemed medically necessary and potential treatment options if available. They may also recommend certain over the counter medications such as eye drops and allergy medications depending on what the work up reveals.