Ugh. Your situation sounds pretty miserable right now. I hope my earlier post didn't sound too condescending. Should've probably realized you had the basics down already, and if it were a straightforward fix, you wouldn't be asking for input.�
I do know that, even though I'm a doctor, I completely lack perspective and objectivity when it comes to my kids. �So, for me, I still think it would be helpful to collect some sort of data if I wanted to objectively compare things. If your son really has no other symptoms except rapid-onset, severe exacerbations, then that's all you can monitor, right? You had 3 months with no oral steroid use on the leukotriene antagonist, which is a huge deal. �Not exactly enough for a scientific study, but it's something to start with. �If it were my kid, and i felt strongly the first med had caused serious issues, and the pulmonologist had given the okay, I'd try the antihistamine until it proved itself to be inferior. Realizing that the need for a single oral steroid course,especially if it happens before Feb, might be enough to show this. �If we did make it to feb without needing it, then I'd cross my fingers and head into asthma season with this regimen. �
This seriously may not apply to you, but also think I historically may have�minimized my son's symptoms without even realizing it, so I'd just caution you to pay attention to the little things, like exercise tolerance/fatigability when he seems well, or intermittent occasional daytime cough. He may display some less obvious signs of control on a daily basis that you can use to help gauge symptom control. (he may totally not, though).
From a behavioral standpoint, while it's certainly possible the leukotriene inhibitor was doing it, but it's also possible it wasn't, or that the side effects are mild enough to figure a way to cope with. I know there has been concern about singulair, but from what I know the jury is still kind of out on it. So maybe a mood/behavior chart wouldn't be a bad idea. Maybe pick one or two behaviors to monitor? That way you've at least got something to compare to if you end up having to try it again.
I did hit on a sort of metaanalysis on the the singular issue. Here's the pubmed link. �I was able to get the entire article, but didn't have time to thru all the details. �
http://www.ncbi.nlm.nih.gov/m/pubmed/19815116/Of course it was sponsored by merck... �Also, it doesn't really matter how unlikely a side effect is. If it happens to you, it may as well happen to 100% of people who take it. The problem is there really are only so many options out there. And they all have risks and potential side effects. It becomes a balancing act, which you obviously already know.
My only other suggestion would be that if the antihistamine seems to make a big difference, you might consider further allergy testing/second opinions regarding underlying environmental triggers for your son's asthma. I may be missing something, but if it's responsive to this, I'd think that would indicate a atopic component. Right?
Once again I'm not sure any of this is helpful. If not, then just think of it as a post of support. Good luck again, with your older son and the baby (and yourself!)