Splinter assumes Leo is dead, and so the boys just grew up assuming the same. Although Leo was hardly one when he went missing, they still honour him like a fallen family member. And the boys talk to his shrine when they feel sad or alone. (Or if they get in trouble, they just say 'Leo' told them to do it)
Sometimes I remember Khary Randolph mentioning in the Turtle Tracks interview he was in with Emilio Lopez and Lloyd Goldfine that he's been trying to get more directly involved in the IDW TMNT comics for years, yet they more or less keep telling him "maybe/we'll get back to you on that."
Like it really is a wonder why IDW hasn't taken him on as one of their primary artists because what work he has done thus far has been PHENOMENAL (never mind that he worked as a production artist for the entirety of 2003's run, so he already has plenty of experience with the turtles.)