I'd think that would be pretty much impossible.
Smell and taste have no real physical attributes for you to describe, like you could describe and object to a blind person. EDIT: with the exception of colour, of course😱😀
Something hit my friend's nose when she was young, and she can't smell anymore.
She judges food completely by appearance, so if the food tastes delicious, but looks bad, she won't eat it. Presentation and appearance is everything for her.
Perhaps you could relate taste to smell. Something that smells horrible, will most likely tase horrible (a few things come to mind...) And something that smells sweet, will taste sweet. That is the colsest you can get. Either that or give him a tongue transplant.
He has a blind girl hold a hot potato for "red" and an ice cube for "blue"
I think there are ways to convey at least the concept of color or taste.
texture and sensation are part of taste. So if they wanted to know what Pop Rocks tasted like, have them hold a sparkler or feel the bubble spray from a freshly poured soda.
It's going to be pretty hard, seeing that they have nothing to compare it to (assuming they're born blind/tasteless). If they lost those senses during their conscious life, then it's a bit easier