Feed store personnel are notorious for giving bad chicken advice.
There are several broad spectrum wormers you can use, none of them are cheap. Most are cattle or sheep wormers and you just adjust the dose for chickens.
I like valbazen (albendazole). It treats liver flukes, tapeworms, stomach worms, intestinal worms and lungworms. You give 1/2 cc orally, undiluted for standard birds, 1/4 cc for bantams. The huge bottle I got cost $35, but it's good for a long time. Draw the correct amount up into a needle-less syringe. If the roo doesn't want to open his beak for administering just tug gently on his wattles, the beak will open.
You can also use ivermectin pour-on (also a cattle wormer). You apply 1/2 cc (1/4 cc for bantams) to the back of the neck, on the skin. Be sure you get good skin contact.
With the roo it doesn't matter, but if you treat hens with either of these it's recommended that you discard the eggs for two weeks. Albendazole is what's commonly used to treat humans with worms.
Some folks say that to be totally safe in case of a suspected heavy worm load, you should first worm with the wazine, wait two weeks and then hit them with the broad spectrum. This is to prevent the dead worms from jamming up the chicken's system and killing them.
If he's with your flock it's probably a good idea to worm everybody.
If you happen to know anyone that raises cattle, you might ask them for a syringe full of the ivermectin. That's where I got mine, from a cattle farming neighbor.
