I'm trying to locate missing alumni for a high school reunion and came across an adoption registry a few nights ago. (I read every single entry for California in the hopes i would be able to help someone.) There were people who had been adopted, given their child up for adoption, and people like you looking for siblings. Basically they listed as much information as they had as possible: name at birth, date of birth, city or hospital of birth, (even on adoptions that took place after birth as he may have that information) birth parents names, ages and where they were from, adoptive parents information and personal information such as their reason for searching (to give or get medical information, longing to be reunited, etc.)
If you know where and when he was born you should be able to obtain a copy of his birth certificate, unless it was amended due to the adoption. Contact the county of his birth for the cost and they will mail it to you if you can't go in to pick it up. If you know the name of the family he was adopted by you can try searching for them online. Since you know his name at birth and date of birth search Ancestry.com, I am working on my family tree and have found information through public records there on people my family hasn't seen in decades. Obtain as much information as you can from relatives or friends, whomever has any knowledge of particulars of the adoption, you never know what will help. Go online to look for Adoption Registries for the state he was adopted in (even if you believe the family moved out of the state afterwards) and enter everything you have, he may be looking, too. Use as many registries as are pertinent, he or someone else who knows of him may use one but not all. If you know or find out his adoptive name do internet searches whenever you think of it, I have found info that wasn't there the day before : ) Hospitals won't be of help if he wasn't adopted at birth, and with the HIPAA laws they may not be able to give any info out at all without a court order.
Best wishes to you, i hope you find him ~ but be prepared in the event he does not feel the same way you do and desire a reunion, it happens that way sometimes.