Is he happy with the options he has chosen for next year? A lot of children do settle down a lot in yr10 - it's partly having ditched the subjects they really hate, and partly just growing up a bit.
He's probably sick to death of the same old stories/reasons from you and from his teachers; so I would enlist the help of all and sundry - especially males - whenever you can. Do you have relatives or friends that he looks up to and admires? Interrogate your friends, and see if you can find people to tell him the following stories. (If they can act, get them to make it up!):
"I didn't work as hard as I could have done at school, and I didn't get in to the courses I could have done at Uni, and because of my idleness I ended up working ten times harder later on to get where I wanted to be. ..."
and
"I didn't really enjoy school, and found a lot of the schoolwork pointless, but I realised it just had to be endured and got on with; so I knuckled down and got the grades I deserved. My family were really proud of me, and it meant the next stage was plain sailing ..."
and
"I used to muck around at school, and play the fool, and distract the other children. Then one day we got a new teacher who I really got on with, and he sat down with me and had a chat, and I suddenly saw things from his perspective, and realised what a pain I was being, and how my behaviour wasn't fair on the other students in the class who wanted to learn ..."
Do bear in mind too that if you're going to choose a time to rebel a bit then yr8/9 is not a bad time. Stress to him that next yr his GCSE courses are starting, and it is getting serious, and he must knuckle down and get the work done.
Finally! Try and relax a bit - especially if it's causing probs with your dh. Ultimately it would be great for him to learn a lesson that he has the responsibility for his learning, not you, his teachers, or anyone else. And plenty of bright boys do muck around at school, manage to scrape through with 'just enough', and go own to succeed and achieve great things in the future.