One of my DCB's is like this since his mom got pregnant. It also coincided with him getting older/more aware (18 mos) so it could have been a result of more awareness that mom was not with him.

His parents want him with me fulltime during her mat leave and I don't know how he'll do. It is one thing to be able to tell them that "mom is at work" but for him to know mom is one street over with his new sibling, might be hard.

For you situation I would replace with a full timer. This gives you an out as he's not adjusting and hopefully when baby #2 needs a spot you have one but there wouldn't be hard feelings if there isn't.