I'd maybe looking into getting a worm count done, that way you know whether he needs it and what he needs. currently there is a huge thing about not worming for the sake of worming as this has left a resistance to the old fashioned wormers. 

It won't cause him any harm to switch.  My horses are on a programme devised by our vet practice for horses in livery, and they have a five day wormer twice a year (panacur guard) and are given several other different wormers throughout the year including equivalan and equest.  Sometimes sticking with the same thing over and over can cause them to become resistant to the effects of the wormer.