The starting point is knowing what state he was convicted in, because inmate search systems are organized by state DOC. If you know the state, go directly to that state's Department of Corrections website and use their public offender search. Most state DOC sites let you search by name and date of birth and will return his current facility, his inmate number, and sometimes his projected parole eligibility or sentence details.
If you are not sure which state he is in, InmateAid's inmate locator can help you search across multiple states. Start with any state you believe he had ties to or where you know the charges occurred and work from there.
Once you locate him, getting back in contact is straightforward. Write him a letter at his facility address, including your return address or send it through InmateAid if you want your home address kept private. Introduce yourself, explain that you lost contact and have been trying to find him, and give him your current contact information. People serving long sentences, particularly life sentences, value reconnection with family more than most people realize. Years of distance does not mean the door is closed.
If you need help locating him, send InmateAid his full legal name, approximate date of birth, and the state or states you think are most likely. The team can help track him down.