If it was really the caffeine, I would suggest that you have his adrenals tested first - if you haven't lately, that is. Caffeine works on the adrenals and "boosts" them. I have serious adrenal issues, and caffeine is VERY helpful, but, in order to heal them, I can't have caffeine as it pushes them into overdrive all the time.
The other possibility that comes to mind is the sugar (unless you've seen him consume sugar without the caffeine and not seen this response). The sugar gives us energy - granted, we usually crash afterwards, but if he has a fungal/yeast issues that are overwhelming his system, it would 'feed' the yeast/fungal issue, and 'relieve' his symptoms of die off for a bit.
Those are the only two things I can come up with as possible explanations this morning. LOL! I need some caffeine!! Ha ha ha!
Herb only treatment for Lyme & Bart ended 12/11 - no active symptoms for 2 yrs -Herb only treatment for Babesia ended 12/12www.healingwell.com/community/default.aspx?f=30&m=2977364
Had Lyme, Bart, Babs, RMSF, Ehrlichia, Myco, Anaplasmosis, EBV
New Lyme case 8/2014 - Healed 1/31/15 - unknowingly had Asymptomatic Babs and Asymptomatic Bart, being treated now though (2/2016)