Soaking in sugar water is only to help reduce swelling to see if the prolapse will go in itself...if it doesn't within a very short period of time..like 20-30min, then you need to take it to a vet.

Prolapsing rarely is caused just because the gecko is excited at smelling/seeing a receptive female, often it can be due to constipation, parasites, weakness of muscles or other issues. I suggest you find a better qualified vet or insist your current vet do a full examination of your gecko, including fecals (bring recent poop sample for this) and x-ray to determine the cause of the prolapse. If he seems clueless on how to treat advise him to contact a more experienced reptile vet.
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PHLdyPayne