how often to worm a horse?

Yep same amount, we worm ours every 3-5 mths.

Do you care about this horse? Are you REALLY comfortable taking advice from strangers with dubious credentials off the 'net?

Just jamming dewormer into a horse's mouth isnt' the correct way to go about it. You must deworm for the parasites present. Fenbendazol and Pyrantal Pamoate have been causing resistance to some parasites in some areas. The only way to deworm properly is to perform a fecal flotation test and find out what parasite ovum your horse has. Then, worm with the medicine that will kill THAT parasite.

Look, you wouldn't throw a t-bone steak into a hamster's cage and say "Oh, I've fed him. He's all set". Because you hopefully know hamsters aren't meat eaters. Same goes for deworming with any old product.

And so far as the timing and the dosage, you REALLY need to take the advice of an equine veterinarian. While you're at it, get his teeth checked, vaccinations updated, and his diet evaluated. Growing horses have special needs.

Be smart about this.