He needs an O-flute bit. These are essentially a single flute that is very wide, wraps around in a slow spiral leaving it open to clear chips. They work well in plastics and aluminum, though there may be different specific geometries for each? Shouldn't be hard to find.
I use both 1/8" and 1/4" styles.

I have read that using 'cast' acrylic is much better than 'extruded'.

I’ve tried my router on acrylic, and the blade speed is way too high. 

Some drill presses have bearings sufficiently robust for this type of work, but a milling machine is the equipment of choice.  Probably with a bit speed of 200 to 300 rpms. 

As a side note, I have cut plexiglass on the table saw using an all steel plywood blade.  It probably had 150 to 200 teeth.  The key was to mount the blade backwards.  I don’t recall where I saw this suggested.  I used that method for the vinyl soffit material and the cuts were super clean.

But I have never tried cutting dadoes in acrylic, so I cannot comment on that. 

I did flame polish the edges (you can find videos online how to do that).  It left the machined edges as smooth as a plate glass.  And it only takes seconds to do.  So a good technique to learn.