@Kyoko I don't think so. Iruma and Ameri are only about a year apart. I think Ameri and Henri just have the same 'type' in terms of appearance. If Henry thought Iruma might have been 'Merize's' child, I think he would have tried to get custody of him, not send him back to the human world. He seems to me to be the type of husband who would accept any child of his wife's as 'his' own. Also, Merize doesn't seem to be the type to be so cruel as to force her toddler to live in a tent while she lives off his suffering. An interesting note is that her hair color changed, which means she either lived a very long life or isn't entirely human anymore.
@Aimz yeah i noticed the changed of color... and i thought she may be get older π€
About Iruma... it could be his "parents" are not his real parents. Maybe we will learn that there will be an accident and Iruma has been lost and in the end of this bad couple "parents" π€