my sons cat is chaseing harry my young cat?

hi my son moved in with me and my cats about 6 weeks ago, his two boys are fine with all my cats toby is getting along great, started to go out (they were both indoor cats before coming to our house we live in a very safe area so can go play in the garden and field) and enjoying his new life. tyler however seems to have a problem with harry, hes fine with all the 6 but he chases harry out the house, harry seems a little frightened of him tyler is a big cat, harrys only a year old, all have been fixed. im not sure how to handle this we think harry must have jumped on his head when he first arrived, harry does that to my cats to play. any advice would be gratefull