My dog suddenly ate a chocolate lava cake in my absence. What can I do to prevent him from any future digestive issues?

The small amounts of chocolate ingestion should not possess any sort of problem. Make sure he isn't lethargic or showing any other symptom. However!It could still predispose to some digestive upset in some cases (vomiting, diarrhea) and some pets show no issues. So you need to watch out for a day and decide to get him treated by a veterinarian if required. Right now provide him more water. The treatment can be given accordingly if he shows any symptom.