I expect that a good therapist would want him to experience doing without reassurance so as to wean himself from it.

You yourself could tell him that you are not going to be his reassurance device.

His comments along the lines of “your silence speaks otherwise” strike me as offensive. He is suggesting that you are not being truthful with him.

It sounds like it would be exhausting to be jumping through his reassurance hoops indefinitely. But does he require reassurance in person or just by text? It sounds like he does better when with you in person.