If your collared is looking dehydrated or thinnish, then NO you do not want him to brumate (at least not right now) I understand your concern because some have a mind of their own and no matter what ya try they slow down around this time of the year without our help.
How is his weight , is he real thin? If not and he looks to have meat on him , just try HYDRATING him, get some liquids in him, and if he continues to WANT to brumate just allow him on his own for awhile in his tank, (See what he wants to do) but monitor him ( as far as weight / water needs. Always keep a shallow saucer of water in his area.
Sounds as if your doing a great job (watching out for him) 
Hey we can not FORCE them to stay awake other than providing the heat/lights they need, so keep an eye on things and make sure he gets a drink each day tierd or not
until he looks better.
Maybe others have some ideas they can toss yoru way!
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PHEve / Eve