"Give me your size and I'll get them while I get your other things."

Does he want to go dig through a store for an item he's not particularly familiar with while he feels like this? No. Does he even consider not insisting he do it? Also no.

He does break from pulling David's headache to throw off just a smidge of his own before he wears out. And a little more than he means to slips, indicating he should stop soon lest he risk transferring all of David's headache.