Yes...I would start confining him again at night...and make it a point...when you get up in the morning to let him out..say "lets go outside to potty" and take him right out and make sure he goes...I wouldn't just trust that he will do it...Keep in mind...Yorkies don't like getting their feet wet..and the grass is wet in the mornings from the dew
..they will potty in the house even if they are fully pottied trained before going in wet grass..My 2 hate hate going in the wet grass in the mornings. I have to make them go in the grass in the mornings to potty...so that might be what is happening with your little guy...heaven forbid if they have to get their feet wet LOL.
Sometimes I find little surprises on my brick pavers right by the grass
...thats how bad they hate going in the wet grass in the mornings...Every Yorkie I have ever owned does not and sometimes will not go in wet grass....Just a thought.