Personally, i do not think it is grooming. Grooming is when someone builds a relationship, trust and emotional connection with a child or young person so they can manipulate, exploit and abuse them.
In this case it is different, because the male lead never had the intention to exploit or manipulate the guy. Although, the weird thing here is that he basically watched him grow up from being a child to an adult.
that's what ticks me off too because he said he liked the uke from a long time and if I remember correctly he raised them for over a decade now (10 years) that means he developed feelings for the uke when he was either 12-22 (I'm guessing the mother died at 12 just to be sure)
as cute as it is ain't he technically groomed??
and to add to that the issue with the guy who murdered his mother is still unresolved