How can I stop my puppy from attacking my cat?

Its more so playing. Thats what are trainer says but my poor kitters is being slobbered and sat on by my puppy. He is almost 5 months old (he is 30 pounds right now) I always have to brush my cats head because its spiked up with dried dog spit. I am scared that one day my puppy will bite to hard and do some damage. My puppy is violent and agressive. Just...a little to playful.