don't worry too much! William sounds like a clever wee bub to me and her certainly gets a lot of stimulation from you...
does William like tummy time AND being on his back? i am certain that Ethan only rolled to get OFF his tummy
why don't you try getting William to play lying on his side? its a good way to bring both hands together to examine objects that are still resting on the ground and it is another position to explore and its halfway to rolling over....
also, i don't know if your HV gave you any advice on HOW to roll William but i do this with Ethan by picking up on leg and bending it at the knee and hip and bringing it accross his body and pulling the other leg through... that way Ethan had to roll his top half whilst i rolled his bottom half, if i started with my hands crossed over we could roll over and back all at once (which Ethan used to love fast back and forth on the bed!)
can you take toys and get him to follow them and then put them out of grasp unless he rolls and reaches and guide him through this and praise him when he gets the toy?
as for sitting, sit him on your knees facing you and bounce him sometimes, this is usually lots of fun for babies and helps them learn how to make muscle adjustments to keep balance.
HTH! or at least gives you some new things to play at!