15 mo and drinking just seems an inconvenience for him. He’ll have sips of water throughout the day (2-3oz max) and 10-12oz of milk which he will only drink if distracted or he gets bored and runs off.
We had a lot of feeding issues when he was newborn / a few months old that were resolved but he’s never really drank much.
Absolutely loves his solid food. We do usually offer him cereals with milk, yogurts custards and jelly’s so he does get hydration from that too.
Just feel like 12-15oz of fluids doesn’t seem much!