The direct challenge to the faith of a JW is seldom effective. Most of us here do remember a time when we would have reacted exactly the same to anyone criticising the Watchtower. For one thing it puts you in the defensive, as it is a criticism of choices made. It also can arouse fear of "apostate thinking", and nobody can think or make good decisions when afraid.
A more indirect approach is usually advisable. You can say someone you worked with asked a question that you don't know the answer to and wonder how they would answer. Of course since you have already disclosed that you yourself have doubts it will be more difficult. I recommend dropping the subject for at least a few days or weeks and try again with a different approach.
You could just say that you have troubling questions that you cannot get out of your mind and you want to know how to deal with these questions. Of course he will spout the usual nonsense that we need to have faith, or trust the governing body, that gives you an opportunity to sow a few seeds of doubt. The objective is not to school the person in TTATT in on e session, but to get them to think independently in some small way.
You might think not about thinks you have trouble with, but what you think they might have trouble with and start with that. I can guarantee that the majority of JWs do not completely buy the Watchtower explanation for everything. For some people it's the blood issue, for others it's the false predictions, for others it's the exhausting schedule of meetings and field service. Figure out what makes him uncomfortable and exploit it.
Another approach is to get them talking about what their life might have been like if they were never JWs. If at all possible do this naturally in the course of a conversation. Did they give up educational opportunities or a good job for the religion? Get them to think what might have been different.