You should tell your professor exactly what you are telling us. Maybe suggest your professor to write it now but hold on to it till needed. Or wait till it gets closer to the time you will be applying.
Regardless of the matter, make sure you give them at least a month notice to write one (Keep in mind that PTCAS cycle opens up at the beginning of July) . Provide your resume, personal statement, or other materials so they could use it as a reference. Make it easy for them (e.g. provide stamped enveloped if LORS are needed to be mailed).
In addition, make sure they keep an electronic copy of the letter they write. This is important. Every DPT application will request LORs. It would be an inconvenience for them (and for you) if they would have to start over every time you find out you need one.
Lastly, try to shoot for 4 total LORS (2 from professors, 2 from PTs). If you have to ask for one, don't be afraid to say, "Do you feel like you can write a favorable recommendation letter for me?" If you get one letter that is not in favor of you (which has happen to students), your application will be turned down.
Good luck!