He very well may be doing other commissions before yours. Working artists who take commissions may have a waiting list of commissions they have accepted, and they usually do them in the order they received them, unless the client makes a different arrangement with the artist, like paying more for a rush job to be done by a certain date. What probably is going on, is that he is working on the commissions that he took in ahead of yours. You did say he is popular, so there could be quite a few, so you will have to be patient. I suggest you email him every two weeks asking for an update, but don't be nasty about it, or he may reject your commission.
Question: Did you pay all the money up front? If so, that was a big mistake. Never pay the whole bill up front for work that hasn't been done yet. In the case of art, pay half up front and half when the job is done. With contractors--like for work on your home or biz, break it down into more payments. Say, 10 or 20 % up front and another 10 or 20 % when that amount of work is done and again after each 10 or 20 % is done. This reduces the chance that the contractor will take the money and run, leaving the work still needing to be done, but no money left to do it.
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau