My daughter also started doing this at 2.5 and the hardest thing to hear is it is a phase that they will outgrow because all you want to do is help them with it right away.
We waited and when she was still doing this at 4 1/2 we finally were able to have her evaluated by a speech therapist. She tested well above age level for everything and the stuttering, it turns out is because she was thinking way faster than she could get her words out.
The reason they don't want you to finish the sentances and/or tell them to slow down when they are so young is because they are still building their vocabulary and it could stunt them if you do this. After 4 years old you can tell them to slow down, take a breath, and then continue. This has worked wonders for my daughter who did not end up needing therapy, and her stuttering only happens once in a blue moon now.
It may just be a phase, sojust listen for it and track how many times it happens. If he is still doing it in a year, talk to your doctor about a referal for speech therapy.