The past 3-5 mos has been winter, if you're in my hemisphere, and just for entertainment sake i'd bet that in addition to being on the cool side he's also eaten less than he did last summer AND i'd bet your house/the room he's in is cooler than it was last summer. Do i win the lottery? 
anyway, i'm guessing you're just seeing some winter pause in his activity. Another guess: is he in a room with windows, so that he's been exposed to the shortened periods of daylight? Just another guess, but all these thigns could account for his behavior.
fwiw, i keep my hondos at about 80-81 degrees...that's the usual temp in the snake room so i don't use heat tapes. That means their warm spots AND their cool spots are around 80. I think some people do keep hondos warmer than i do, but it's also possible if you reduced the temps at the warm end to 80-81 instead of as high as 85 you might find he's mvoign to that end too sometimes. Lastly, I'd imagine as the days get longer and everything else warms up you'll find he moves around more.
keep us posted
terry
>>Hi, I have a hypo honduran milksnake for 2 1/2 years now. I don't if this is a problem or not, he has not been on the warm side of his cage (he is in a 28 qt. sterilite box) for about 3-5 months now, he has only bin on the cool side, No it is not hot at the warm end the warm end is about 82-85'F and the cool end of his cage is about 72-76'F. Thanks Scott Glover