Finding someone who is not showing up in a search can happen for several reasons, and there are a few steps worth trying before assuming the information is simply unavailable.
Start with VINELink at vinelink.com. Search for his full legal name, making sure you use the name he was booked under rather than a nickname. If you know the state where he was arrested or sentenced, search there first since VINELink is organized by state and jurisdiction.
If he is in federal custody, the Bureau of Prisons inmate locator at bop.gov is the authoritative source. Search by name or, if you have it, his federal register number.
For state inmates, the relevant state department of corrections website has its own offender search tool. Knowing which state narrows the search considerably.
A few reasons someone might not appear in a search include a very recent arrest where records have not yet been updated, being held pretrial in a county jail that does not publish an online roster, a name spelled differently in the system than you are searching, or a privacy designation on the record.
If you have his full legal name and a general idea of where he is or where he was arrested, email aid@inmateaid.com and the team will do what they can to help locate him. Include as much detail as you have, full name, date of birth if you know it, and any information about the state or jurisdiction involved.
Once you have a confirmed location, InmateAid can help you send him a letter right away.