My dog had to undergo the treatment when he was around 1 1/2 (the good things was the preventative company paid for it because he never missed a dose)
It started with 30 days on minocycline antibiotic, then one shot.
I think it was around 30 days later he got two shots 24 hours apart and stayed at the vet overnight.
During this time he was confined to the laundry room since it’s small and outside only on the leash for potty. No playing, he was kept very calm.
I think (if I remember right) it was about 3 months of quiet rest and then slowly increasing to short walks and eventually full activity. It is very important to keep them quiet. He handled it well
He has since passed on from cancer but he lived 7 years after that. Never had an issue again and was heartworm negative the rest of his life.
Best of luck to you and your pup