EMB145 First Officer reports Captain as flying pilot descending early during ILS approach and getting well below glide slope until questioned.

Narrative

We were flying IMC; overcast at 800 FT AGL. Captain was pilot flying; First Officer was pilot not flying. We were issued the clearance; 'Maintain 2;000 FT until established; cleared for the ILS Runway XXL.' Captain was flying at 2;000 FT; and approach mode was armed. I looked at Primary Flight Display; and saw LOC and GS standing by. Autopilot commanded turn to capture LOC and LOC was captured. We were still below the GS; but level at 2;000 FT AGL as assigned. We were issued a handoff to Tower and I changed to Tower frequency and said we were on the ILS Runway XXL; and we were cleared to land by Tower. I looked down at my approach plate to ID FAF and GS intercept altitude and to determine 1;000 FT and 500 FT altitudes above Decision Altitude. When I looked up; Vertical Speed was shown on Primary Flight Display and not GS and we were descending through 1;200 FT AGL; 800 FT below our intercept altitude assigned by Approach! Soon after; aural warning; 'GS; GS' was heard and illustrated on Primary Flight Display. I told Captain that we were too low and that we should be at 2;000 FT! Captain was confused and said 'What?' and I read the clearance out loud again; 'Maintain 2;000 FT until established; cleared for ILS Runway XXL Approach!' Captain turned off autopilot; leveled off altitude; and hand flew approach. Captain was clearly nervous and task overloaded because I had to tell him; 'Turn left to capture LOC; OK now turn right LOC moving right...descend GS is below; etc....' This went on for a while. Once LOC and GS were satisfactorily captured; I reached up and re-engaged the autopilot; turning it on. Captain immediately turned autopilot off; and hand flew approach until landing. Captain said he wanted to hand fly approach; even though I came very close to having to take controls from him. I called runway in sight '12:30 o'clock position' and Captain flew visual approach to landing. We broke out of IMC between 1;000 FT and 500 FT AGL.