My personal opinion, from personal experience and the experiences of friends and relatives, is that antipsychotics are best avoided except in cases of actual psychosis or extreme aggression.
I've been/seen quite a few people turned into zombies by either Risperdal or Seroquel. My Risperdal horror stories are plastered all over WP; I watched a friend of mine spend months and months and months eating Seroquel, apathetically letting her ex-husband anally rape her in exchange for visitation with their kid. Beyond sad.
I note that both my friend and I were on high doses that we were instructed to take multiple times a day, not low doses at bedtime. There may be a huge difference there.
I won't say don't do it; it might very well help. A decent night's sleep is something everyone needs (and if a 3-year-old isn't getting one, you aren't either).
I will say to be watchful. Very, very watchful. Read the list of possible side effects. Memorize it and be EXTREMELY watchful, to the point that all your friends think you're paranoid. Especially watch for personality changes in the direction of constant sedation, apathy, inability to enjoy anything. Cognitive dysfunction-- if he starts losing skills, drop that s**t like a hot potato. I don't think Seroquel is notorious for debilitating large-muscle pain (Risperdal is; believe me they aren't kidding), but watch. If he doesn't want to move or be touched or seems to be in pain, believe that he is.
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"Alas, our dried voices when we whisper together are quiet and meaningless, as wind in dry grass, or rats' feet over broken glass in our dry cellar." --TS Eliot, "The Hollow Men"