He manages a nod: he can try, at least.

As carefully as he can, Alistair clasps Ysa's shoulder. The grip gets a little less gentle once it's time for him to stand up -- it becomes obvious very fast that he can't carry much of his own weight anymore. Now that he's not trying to hack his way through hordes of demons, his body seems to have taken its cue: it's finally time to rest.

But eventually, he's on his feet again, only shaking a little bit from the effort.