You don’t mention your relationship with this person, but I’ll assume he’s family. What does “meaningful conversation” mean to you? If you are seeking to discuss physics or mathematics, you should reconsider your expectations. But you if want to tell this person how much they mean to you, that you’re there for them and love them, then you should adapt your approach.
First, consider the environment of your visit. Are you in a calm place, where he feels comfortable? Also consider the time of day. People with Lewy Body can have remarkably different abilities at different times. Attempt to find out his best times. Try taking his lead in the conversation. If you arrive planning to talk about sports, and he clearly isn’t engaged, try asking him some questions to see what is on his mind. It might not make sense to you, but it does to him. Read his mood, and respond to his feelings, if you can’t to his words. Maybe the conversation doesn’t seem meaningful, but the moment is. Hold his hand if he seems to need comfort, take him for a stroll if he’s restless. Try to focus your visit more on an experience than a conversation. Bring music or snacks he likes, or put on a musical or movie that’s a favorite. If it’s hard for him to carry conversation, try talking about things you remember that you’ve shared. And sometimes, maybe, there is no need for conversation at all. Just being there is meaningful.