I'm so sorry you're dealing with this. Singulaire made my husband crazy, too--but he's 37, so at least he could talk about it and we could figure it out (although for a while there we thought he really did need to get into therapy!). From talking to friends who are doctors, Singulaire is well known to cause anxiety attacks in some patients. However, there are few effective RX's for the combination of allergies and asthma (my husband's problem, as well). SO, my husband switched to homeopathic and naturopathic (herbs and diet) treatments. He still has some allergies, but they can now be treated with the occasional claritin, plus he can breathe and isn't crazy! I'm sure the doses and such would be totally different for a little one, but here's who I suggest you contact for more information:
1) Mike's healthfood store on the NE corner of 75 and White in McKinney (their staff are very knowledgeable).
2) Angie Shapira at North Dallas Shiatsu (PM me if you'd like email/phone number). She practices naturopathic medicine, as well.
Good luck finding what works for your son!