Because the title of your post is Case DC clutch, I thought you were saying the CLUTCH won't release.
IF you actually mean the gear shift lever will not move from 1st to neutral----IF your dad meant it finally did move to neutral at a later time once before--- Because it is winter time--because the gears slide sideways in and out of engagement--because it is headed down hill where water in the transmission would collect under the sliding gears--because DCs are prone to collect water in the transmission from many years of setting outside. MAYBE YOUR GEARS ARE FROZE IN ICE.
One of the DCs I restored was in a barn in the winter time when I went to get it. It would not roll because the gears were stuck in ice.
The 49 DC hand clutch runs in oil when things are right. IF your clutch is stuck engaged, something is dry and needs oiling. Take the plate off the left side of the clutch and inspect. If things look dry, get an aerosol can of roller chain lube. Douse everything inside a time or two and let it set awhile.
DCs are prone to have a lot of sludge collected in the bottom of the crankcase. If your clutch is the problem, it may be caused by engine sludge getting into the clutch. Do your tractor a favor and pull the side plates on the engine and check for sludge. Also clean the oil pump suction screen.