if you are sure he has a large worm burden, then you are best to use something like panacur guard and do a lower dose over 5 days. Its much kinder to the system.
What ever you do, do not use paramox, this is the opposite and a "good strong wormer", so could cause colic, bleeding and general upset if the huge worm burden is flushed at once.
I would ask your vet, they will be able to give you the best advice, and advice over the phone is free! I would have thought panacur would be the best thing though possibly?
I asked my vet about worming 2 newly bought and very wormy New Forest foals. He suggested one of the older brands oF wormer, think it was Eqvalan as he said the newer ones could make them scour.
That wormer produced shed loads of worms in the poo, it was GROSS!!
Identified some: round worms and bots, and several other types I wasn't sure of
My foal came to me at 9 mothes old she was very poor and we supected a high worm burden was advised to use panacure, the following morning was like a horror movie!! manly round worms lots yuk anyway she was fine no ill affects , however a friend bought a older pony which she wormed the pony soon became sick seemed collicy she called the vet who was worried it might be grass sickness, wasnt eating or passing anything anyway pony did get better after a few days pasing a lot of worms , maybe worth getting the wormer from your vet they will advise on a safe worming program at the same time, good luck with the foal..