I don't know from personal experience, but I know there are various protocols, depending on the load of the heartworms (how many adults are already in the heart), the age of the dog, the overall heath of the dog, and whether or not the dog has symptoms.
I can see waiting a little while if the dog is undernourished or otherwise weak (from other things), but the longer they wait the worse the HW load will get. They do have a cycle, tho...and die off naturally. But if adults are forming daily, wouldn't it be the same 6 months from now? I really don't know. I do know most want it taken care of right away if at all possible. Is he giving any sort of treatment for them on a daily basis? Some Vets do that too...treat daily for "x" number of days. They're using a weakened dosage of regular Ivermectin, so the kill off is minimal yet no new larvae can form.
They must be careful, if there's a big adult HW burden...to kill off the HW's slowly so they don't all die quickly and block the heart.
I found out the treatment my son got for his dog (a young Lab he rescued) was the Immiticide treatment. Sometimes they do a round of Doxycycline prior to the Immiticide treatment, as well.
IF she didn't have a huge burden of adult HW's...she would have gotten one shot in the left hip, then 24 hrs later, gotten a shot in the right hip. Then would have been put on Heartgard (Ivermectin) for the larvae in the system.
HOWEVER, since she DID HAVE a heavy burden of HW's....they gave her a shot...waited a month, and then gave another shot in one hip, then a third 24 hrs later in the other hip. This slows down the kill off and is less risky for the dog.
Then they put her on Heartgard (or Ivermectin) once a month for the larvae.
There are also Holistic methods that I've heard work...some mixture of Black Walnut, Cayenne Pepper, Rosemary and Garlic...but a Holistic Vet would have to make the mixture as Black Walnut can become toxic over time. This is something they use in HI for heartworms.
And...another Holistic treatment is Paratox, Ionic copper and HW nosodes, which is a gradual process over months.
I'd really have to question the Holistic Vets about the success rates of these plans, tho.
There is no "preventative" for HW in reality. The pill (such as Heartgard, Interceptor, etc) they get actually rids the dog of any larvae or immature HW's before they become adults and the cycle starts .... if any exist. That's why they want a HW test before starting any dog on "preventative" because if they DO have adult HWs the "preventative" could do a fast kill off and cause a blockage in the heart. Often if they find a very "light" positive, they'll just treat with the HW pill...as normal and recheck in a month.