It has become rather fashionable among intellectuals to deny free will, hasn't it? Like Charles, I have a friend or two in that camp. Let me rephrase the syllogism in a way that balances the terms;
P1. Pleasure is the end (telos) in nature toward which human life is observed to incline.
P2. If there is an end observed in nature, then the decision to pursue that end is predetermined.
C. Therefore, the decision to pursue pleasure is predetermined.
It is a rather thorny question. Does the syllogism as I have expressed it assume (in premise 2) what it is asked to prove (in the conclusion)? In other words, does the second clause in premise 2 follow necessarily from the first clause? Could I not, for example, choose to pursue an unnatural end? Instead of a choice between pleasure and displeasure, could I choose a third way in which pleasure doesn't factor? Not because I expected a greater pleasure from this third way, but just for some other reason?
Another way of putting it; suppose I granted that pleasure as the telos provided a rationale for my decision; am I also granting, ipso facto, that my decision is predetermined? I'm not so sure.
To put it simply, I suspect that our imaginary interlocutor is begging the question.