help devising taper from 4mg diazepam;

I need help figuring out how fast a taper for my son to get off his remaining 4mg of valium.  He has hit a wall;  he has bipolar disorder, and is on other meds, too,  but I think the psychiatric symptoms he is having is from being tapered from 15 mg diazepam to now, with recent cuts being too fast.  Or it is from being in constant withdrawal tolerance.  He has been on benzos constantly for 2 years, with Max of 15mg diazepam equivalent.  He has made his cuts 1mg at a time until now, and splitting a 2mg pill.

Questions:

1)  how long to stabilize now until he starts a taper again?  Still having some withdrawal symptoms of overactive mind, depression;  wait weeks?  months?

2) how to proceed with the next phases of the taper;  How long to plan to get off the rest?  What percent should he do?

I don't want to stay on longer than he needs to, but also he is very sensitive to med changes - so need to go slow enough to try to keep his bipolar mania in check.

We have obtained a blender, a 250cc cylinder, a mortar and pestle.

What do you think?