Alchemists specialize in transference and changing one thing into another. It is a much riskier version of arcane magic, but also much more stable. It would certainly be possible for him to use alchemy, and absolutely possible to do so in the way you mean, especially if those bottles contained 'essence' or basically an artificial potion version of whatever he wants to transform into. In that regard, he would basically be transferring the energy and essence from the bottle into himself to cause the change, and then ending it by reversing the process. The danger in that would come from the potential to become trapped, temporarily or permanently in a form due to the destruction of its vial/bottle or corruption of the energies involved.

If that sounds workable/fun to work with, we say go for it! We do recommend picking Alchemist as a racial trait if you are interested in him being able to acquire additional future 'forms'/bottles in game and be able to use them immediately.

This would be shape-shifting, just in a more artificial form.