Your tailor shouldn't try to alter the shoulders. What he may be able to do , and may help, is release the back seam a little in the yoke area, to give you more room there.
In the future, you should establish whether you like the shoulder and chest fit before you have the jacket altered. That's true across all brands, styles, fabrics, etc... There are always variations in both exact measurements and fit due to fabric properties - so you may love one jacket in size X but hate another one in the same size but different fabric.