You mention him not taking on the parenting role. Well...he's not their dad and in this situation--even experts say this--it's up to the parent to do the disciplining. Actually, his response to your kids---go ask your mom---is the appropriate one in this situation. If you were married and he adopted your kids, then that would be altogether different. What---IMO---would be best would be for you to tell him in advance, for example, the kids can have a snack after school and what is an appropriate snack. That way he can say that you told him what was acceptable. Otherwise, it's not his place to set rules on his own.