Sounds like he could be slowing down for hibernation. Arg B&Ws hibernate for the winter, even young ones. They dig a burrow and will remain unactive for 4 to 6 months. They do lose wieght but then they have one hell of an appitite when the wake up.
Has your tegu moved away from his heat sorces and is less active now? Also what are your cage temps and what are u useing as bedding?
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Leo Gex
1.0 Giants
0.1 Red strip/hypo
1.0 Patternless
0.2 Normals
Others:
1.0 Giant plated lizard
1.0 Blue Tegu
1.1 dogs
2:0 chinchillas