My son also had a problem metabolizing too fast, but it was with Adderall. We had a similar problem with having to up the dose so high that we weren't comfortable and he started having the "zombie" type side effects. When we switched to the "XR" (extended release) version of Adderall it got better but not great. When we switched to Ritalin it was MUCH better. He doesn't metabolize it as fast and he's much more steady.
I guess the point to my post is that one medication can react totally different than another even though they are all called "stimulants". There IS a med/dose out there for you...it just may be frustrating finding it! Good luck!
So we are going to take the plunge....We're taking the plunge right along side you 
Have been having the most awful tics with Vyvanse and have just gone back to Concerta (which has tics as well from past exper, only maybe not so bad)and just added the first dose of clonidine (which I think is pretty much like tenex)to help with the tics. We tried the Focalin, but it was about like giving an M&M...no effect at all. I know what you mean about "puts the 'H' in ADHD" LOL! I say the very same thing! I'm sorry I don't have any helpful advice...just someone jumping in with you - will hope that it works for both boys!
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8 y/o son, ADHD
everything in the pharmacy lately,
36mg Concerta and 1/2 .1mg clonidine tid today....
We are ending the plunge. He's been taking the concerta since 7/31, at increasingly high doses, without so much as a calm moment.. We are afraid that he is metabolizing it too quickly for it to do any good at lower doses......