Hey Bexj, sorry to hear about your poor moggie.
I have three bengal boys and one moggie. Generally they all get along just fine however my young boy Lenny (who happens to be the biggest) has got into a few similar situations with my moggie.
In my experience there has been 2 reasons for this (a) he has been a bit bored and looking for trouble
or (b) and probably more likely, he starts off as boisterous play, my moggie takes it really seriously and he ends up retaliating.
My solution has been not to leave my moggie with them unsupervised (which is fine for me as I have a fairly big home). Also keeping young Lenny stimulated keeps him out of trouble.
My moggie is fairly young and gives as good as she gets to be honest, I can see your dilemma with an 11 year old. Have you tried using feliway diffuser - worked wonders for first three months I had the boys home - peace and harmony prevailed