you could try toast fingers, or breadsticks, raw carrot / veg
things thats are harder to bite so he can have a sook on them instead..might get him used to bringing things to his mouth, and tasting etc.
you dont need to worry about them not being 'proper' food yet, as he wont be eating anything yet...
rice cakes or other wee organix snacks would be good too as they just turn to nothingness in their mouths, so theres literally nothing there to gag on