For myself and many others, Seroquel, and any atypical antipsychotic meds, actually makes the withdrawal worse. It can really exacerbate RLS. It's a very potent medication, with a LOT of side effects, and can cause a lot of problems. Just make sure it's absolutely necessary for him to take this if he does, whether now or once he's done kicking. Be very careful in weighing the costs and benefits.
It's pretty rare that it really worth taking, especially without a specific kind of condition it's indicated for. Insomnia, anxiety, these are not very good reasons to take it. There are a lot other much safer medications that work better with fewer side effects than any atypical antipsychotic like Seroquel, Risperdal or Zyprexa.
The Alprazolam/Xanax will not blunt the buprenorphine. Definitely increase that and significantly decrease if not discontinue the Seroquel. Alprazolam will not cause him any problems during acute withdrawal, and if anything it will make it a lot easier on him. It would be better to use clonazepam/Klonopin or daizepam/Valium or even temazepam/Restoril, but work with what you have.
Normally day three to four is the worse, but because he's been taking buprenorphine it shouldn't be so bad since it is in his system now. Even though you don't have much to work with, and this is regardless of how you dose it (whether in a couple bigger doses or many smaller doses I mean), just make sure he continues taking enough buprenorphine to be comfortable. I would think this will be about 4mg per day at the least for now.
If he still has the runs really bad, increase his dose of loperamide/Immodium. There is no downside to that medication right now.
As you should know, he should really really get rid of whatever he found. It will do him no good. If he uses it now, he won't get sick. Just as long as it isn't a huge dose, it won't even affect him much because the buprenorphine won't allow it to properly bind to his opioid receptors. At best it might help with some of the physical or GI side effects of withdrawal. What I'd worry about is if he doesn't take it now, saves it until he stops taking buprenorphine, and once the bupe's out of his system and no longer blocking his opioid receptors he takes whatever he found and gets fucked up. I mean, it wouldn't be the end of the world, but it also is a very bad idea.
One of the hardest things for me to do in acute withdrawal is to get outside and get even a little exercise. Encourage him to go on a walk in the morning and evening, and go out with him if you can because it will make it easier for him. It's amazing how supportive you are. If only I had someone like you in my life last time I kicked!