That is a good idea, and I'd prefer it, but have to pay upfront for six sessions.
Actually, I think that works to your advantage. Six sessions is enough that one bad experience won't necessarily torpedo the whole experience, it's enough that your DS will probably have to give it a legit shot, but it's short enough that your DS can see the light at the end of the tunnel if he really, really hates it.
We've found this sort of thing to be helpful in other ways for our DD, except that these were all in activities that she originally asked for, then wanted to quit. If we paid for the full session, she finished the session. It helped us undermine her perfectionism, because now she had to keep trying, and found success. It also taught her about perseverance and honoring commitments, because in some cases (drama, soccer) others were depending on her to fill her role.
And what often happened is that, after the initial crisis had passed, and she finished the session... she wanted to do it again.
So, "I have already paid for six sessions, at the end of those six sessions, we'll discuss how it's going and decide whether or not to continue" is an approach I would recommend.
