Everyday stressors symptoms and conditions

My son used a nebulizer 2 to 4 times a day every day from the time he was one and a half years old. When he had just turned three his doctor prescribed Singulair. It was like a wonder drug for us! It took care of his asthma and we didn't have to use the nebulizer any more. He's been on it ever since -- he's 10 now -- he also takes zyrtec and has a rescue inhaler that he uses maybe once a week. About every other year he requires a course of steroids and a week of regular nebulizer use. Also, for the past year he has also required a daily inhaled steroid.
Now, about his mood issues. My son has always been sensitive and intense, moody. The first time I became alarmed was when he was 7, and he told me he wanted to burn his hands on the stove to punish himself for forgetting his homework. I consulted a psychologist who evaluated him and said he was not clinically depressed. Since then he has had periodic "dark" episodes -- especially in the winter. He has said he wants to die. He has had crying jags over things that are upsetting (loss of a pet was the worst) but it seems excessive for him to be saying he "just wants it all to end." He has told me that he is always unhappy and that he hates himself. He has also had problems with moody acting-out with friends. He will brood about hurt feelings until he loses his temper and screams at the friend. I have worked very hard with him on learning to manage his emotions. He hit a friend at school who was teasing him. He accepted his consequences willingly and willingly wrote letters of apology -- he told me he thinks he has anger problems and doesn't want to be this way. And his character is that he is a sweet, caring boy who can't stand to see anyone hurt, but also can't stand to be hurt.
A couple of years ago I asked his allergist if any of the meds he's on are linked with depression. He said no. We have a family history of depression, and I thought my son had gotten the worst combo of all the genes.
Recently, this all got to the point that I decided he needed to see a psychiatrist and quite possibly take medication for depression. Before I made the appointment he had a check-up with his allergist. Going down his list of meds the dr. said, recently Singulair has been linked with depression, have you noticed any moodiness or sadness? My first thought was that I have, but that he's always been like this. My 2nd thought was that he has been on Singulair for most of his life. I said yes and that I'd like to try him off of it.
My son resisted going off of it. He has had enough negative experiences with asthma that he didn't want to risk it, but I insisted. I didn't expect to see any change, but I thought it was important, as I was going to take him to a psychiatrist to consider depression meds, to see how he did off of it for a couple of months.
Less than a week later, he had been in a wonderful mood -- to the point of being silly and giddy all evening -- for 3 days in a row. The kind of mood that I don't see him in often, and when I do I think to myself, "he should be like this more often." One evening he even realized he had forgotten to bring home a homework assignment. I thought, "oh no, here we go, his evening is ruined." But he talked through his options with me, looked a little uncertain, and said, well, okay, I guess I'll have to tell my teacher I don't have it. I'll tell her I'll make it up at lunch if she wants me to. That was it! He didn't mention it again. I didn't say anything about his mood, because I really don't think I can know anything after just a few days -- it could be coincidental. The next day, he said to me that he thinks being off the Singulair is "working." He has now told me that a couple of more times.
I am tentative, but amazed. Even if my son does have a predisposition to be depressed, maybe the Singulair was making everything worse, and things really can improve for him. I am afraid to be to hopeful. At the same time, I feel guilty for giving this medicine to him for 7 years without a second thought.
As an aside, my son has periodically complained of leg pains, that I always told him were growing pains.
I would love any feedback that anyone can give me. So far (these two weeks), his asthma has been controlled with pulmacort, zyrtec and albuterol, so that aspect is okay.