DS1 is in year 9, aged 13 and has just had his school report.
Generally speaking, he is a quiet, able child who is pretty clever and is quite happy at school.
His school report is, on paper, very good (mostly A's, some B's,) but I know that he could do much better if only he hadn't inherited the lazy arse gene from me 
DH and I like to give him a gift if we feel he has made an effort, and in some subjects he has improved, so I've bought him a cookbook which I know he was interested in (he loves baking)
I'm just trying to write a little card to go along with it, but am struggling. What I want to say is "well done we are proud of you and what you achieve both at school and home, but here's a kick up the backside and stop doing the bare minimum to get by when you are capable of so much more"
I guess I'm frustrated because i know I wasted my brains at school as I only went for the social, and I don't want him to do the same
So what do I say? Any ideas gratefully received!!