PLT used cannula connected to portable oxygen system during climb starting at 11,000 ft MSL. At FL 170 PLT switched to a mask connected to ACFT oxygen system. At FL 250, PLT complained of stomach pains for aprx 1/2 hr. PLT then stated to his son that his oxygen smelt bad, the mask was awkward to use and it fit poorly. At this time the PLT used the mask and cannula together. The ACFT oxygen system was checked and indicated a positive flow with a bottle pressure of 1500 PSI. PLT then began showing signs of hypoxia and subsequently drifted in and out of consciousness. PLT's son took over controls, declared an emergency and flew the ACFT until final apch when the PLT took control of ACFT. PLT dived ACFT on short final and son pulled back on yoke. ACFT contacted ground short of the RWY. Inspection of the ACFT oxygen system revealed no mechanical malfunction, however, the oxygen bottle reportedly had an aprx dew point of -35 deg F. Oxygen used for aviation is required to have a dew point of -83 deg F or less.