OK, thanks for explaining!
Here are a few suggestions:
"It's no problem," he said sincerely, pulling his hair into an almost ponytail before letting it fall again. Below the surface, he was still burning; then he paled. "I'll check in on you later." --> doesn't explain "paled" (used as a verb) in detail, but gives the reader a clue that he's blanching with fear/worry
"...he was still burning. Then he paled." --> separated the previous sentence at the semicolon for an alternate structuring
"It's no problem," he said sincerely, pulling his hair into an almost ponytail before letting it fall again. He was still burning below the surface, quailing with a pallid fervor. "I'll check in on you later." --> less like your style, but illustrates how his fervor and/or worry makes him fearful
Other ideas for the "burning and pale" sentence:
"Below the surface, he paled even as he burned." --> I don't like this too well but it is what it is lol. If the readers have been following along they'll know he's burning so you shouldn't need to use "still."
"Below the surface, he simultaneously burned and paled." --> (alternate adverb)
"Below the surface, his cheeks burned with a pallid flame." --> more poetic, less literal than the other suggestions; it may be too far off from what you aim to express
Hope that helps!