Your setup sounds good, and wow he's a huge boy if he's only 6 months!! I've got a 19 month old who's just reached 21inches haha.
And the same 19 year old went through what you may be experiencing last year, he wasn't old enough to fully brumate so kind of slowed down, got really lazy, didn't eat for about 2 months.
Every beardie owner goes through the worrying phase first time during brumation haha, I think you're fine, although, I might consider turning his basking temps down a bit, for adults basking would normally be around 95F, but then again Jim sounds like he's enjoying the temps if he's not moving away to the cool side. Just a thought.
Sounds like you're all good!