First, have patience. Second, if you do not have a good thermometer in the cage, please get one. It shouldn't cost you more than $10. If you have no thermometer, or a cheap one, the temps may not be accurate. If it is too low, that could be the reason he is hiding. If it is appropriate, but towards the lower end of the spectrum, you could try raising the temp slightly by moving to the next highest wattage bulb, or adding a bulb. If you're using say, a 100 watt bulb, going up to a 150 may bring the temps up too much, but adding a 25 watt may be just right. I can't say for sure though, and you would have to test things out too be sure.
Otherwise, if everything is truly correct, he may just be one to brumate early. Usually they don't go till a few months later, and some won't at all. However, it would not be unheard of for a dragon to brumate early. If that's what's going on, there isn't much you can do to stop it. Since he has been checked out by a vet and is in good health, there is really no reason to make an attempt at stopping. The case would of course be different if there was a health issue. Then you might be told to keep him awake so that the problem could be treated. That of course would really be up to you and the vet to discuss.
The only other thing I can think of is that perhaps you were feeding him something different, and he is holding out for it. If they like a certain food better, they may hold out to get it. I've already forgotten whether or not you said he was eating normally or just a little bit. If you have recently removed something from the diet, or gave him something new, he could be holding out in the hopes that you will give him the desired item again. This happens with waxworms sometimes. If they get them for a few days and then go back to crickets, they may in a sense save room for waxworms because they are hoping to get it again. If that's it, just wait it out and he'll start eating again.
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3.0.0 Bearded Dragons Woodrow, Dexter, Tehko (My first beardie Griffin who turned me crazy, and Erik my Rankins, who will remain a midget- deceased)
1.0.0 Cat- the regular variety (Dominic- fat cat)
0.1.0 Dog- Lab mix(Betty- escape artist extraordinaire)
1.0.0 Red Eared Slider (Franklin)