Sixty isn't old.
We have many members here in their 60s plus. Is the painting of Mina absolutely complete? No other colours needed to add to it? Leonard should now have five colours of paint -- double check to be sure. The game should have played a tune when you have finished painting Mina.
Then as I mentioned before, some more ocarina and/or grooming him.
Hopefully that might do it? (Crossing fingers.)
If you have anything else in your inventory, try giving it to him, you never know, he might like it.
and keep us posted.