I have had a high billiruben count up to 4.3 since I was about 12. I am 34 now, and the most common "symptom" has been yellowish eyes. I also get the jaundice looking skin too.
But other then that, no real symptoms at all. One thing you have to watch out for is doctors who do not understand the disease. I had to go through colonoscopies and barium enemas because they thought I had internal bleeding because of the high billiruben count. So just make sure they are aware of it if he goes to a new doctor.
When I notice my symptoms getting worse, it is because I am not drinking enough water. If I drink my 8 or so glasses a day and stay off drinks that help dehydrate you (cokes and coffee), then I am fine. The water helps flush out the billiruben from the body.
Hope this helps,