I have an older horse with sore rear stifles, looking to give him some extra TLC to get him feeling good. I've been applying some Thermaflex to his stifles after a workout, and will be giving him bute before he starts running again and applying BOT quick wraps. Anything else I should be doing? 

Thanks!

Inject.

Who made the diagnosis, you, vet, chiro, farrier? 

I'd be trying to figure out why he is sore in both stifles before trying to treat.

Lots of time off or you can inject or Prostride them. Or if he's not too sore might try equioxx and see if that takes the inflammation out.

Did vet xray stifles?

Take ultrasound?

Depends if it is just general soreness or if there is something else going on.

My farrier made the diagnosis.

His hocks are fused.

My farrier made the diagnosis.

^^ This comment makes no sense. ^^

Fused hocks doesn't mean they are completely locked in place, or they wouldn't move at all.  There is still a joint there. 

Injections into stifles are a thing as well as hocks. 

Try PHT stifle wraps.  But maybe go to the vet and figure out where he really hurts first. 

Your farrier can't diagnose the issue and neither can the BHW Betty's. . . You need to get this horse in to a good performance horse vet to find out what's going on.