Inigo knows what Roland is doing here, really. As easily as Roland can read him, Inigo can read the other guy just as easily in turn. Especially at a moment like this. This kind of gesture is definitely Roland trying to comfort him and make him feel more at ease in the conversation again - especially with that suggestion he's making to Senku.

It makes Inigo blush a little, feeling embarrassed at having been seen through so easily, and at needing this sort of comfort in the first place - he's old enough that he shouldn't be getting upset about not knowing certain things! ... but at the same time, it really is kind of nice.

Inigo is quiet at first, just listening to the other two talk, but then he finally glances up - first at Roland, then at Senku - as he asks: ".. What's cotton candy?" Look, he's trying to involve himself again, shyness or no shyness!