Excellent question, but I am sorry your kiddo has been struggling through this. It is really difficult to know exactly what is going on over the internet like this (of course), so I urge you to speak to the prescribing physician. But here is the direct answer to your question:
SSRIs (the family to which Lexapro belongs) does not typically have much of an impact on depression in younger children. It can work to curb some anxiety-related symptoms, but not usually depression. Too, in that short of a time (5 months), it would be highly unusual for anyone to develop a "tolerance" to the medication, or for it to decline in effectiveness. Furthermore, it takes about two weeks for the medicine to kick in at all anyway, so it is puzzling that you son showed improvement so quickly - I would be more inclined to believe that the initial two weeks saw either (a) both he and you knew that treatment was started, so you changed everyone's behaviors in the short run just because you all felt that it should be better (this is called the placebo effect). or (b) you kiddo leans more bipolar (you did not mention his mood disorder), and the Lexapro began to kick him into manic phases more rapidly. In this case, the Risperdal would curb some of the manic/acting out tendencies, but is fighting against the Lexapro. It may be necessary to wean him off of the Lexapro entirely in order to allow the Risperdal to maintain its effectiveness.
Again, his psychiatrist is going to be in the best position to evaluate the most effective course of treatment, but I like that you are being active in your child's treatment. I think that once you work out the medication, and if you have a good child therapist to work in con junction to manage the cognitive and behavioral aspects of the disorder, then he will have a good prognosis. If you are satisfied with the response, please hit "Accept." That is the only way I can receive credit for my answer. Thanks-