I'm not sure how useful it is to say something like 'work hard' or 'make an honest effort'. They're vague phrases. I think sometimes a more concrete and structured approach is needed. (If the child is willing as pointed out)
Maybe ask that he spends a solid hour every day working (start small and manageable) in a place where you can see him, not holed away in his room but not somewhere with you breathing down his neck either. You could ask him to tell you what he would be working on beforehand and then afterwards ask him to show his work or ask to quiz him if he has been memorising.
I didn't get to do this with DS as he just wasn't willing in a quiet and stubborn way, but this approach is going ok with DD4(12) who is both lazy and afraid of trying for all the reasons Arti enumerated.
DS went to school with the intention of having a good time, and ended up being voted funniest boy and class member most likely to become a famous hacker. He turned in homework sporadically but studied enough to get the odd B+ or even A in exams. Unfortunately as he was in a system where grade point average is the name of the game (in the US), all that missed homework and so-so results in exams meant he averaged a C+ . He was really good at standardised tests, and seemed to thrive on the adrenaline of exams. The slow but steady 'do your homework daily and do the occasional long term project' was what killed him. School was great - but naturally I only heard about missing homework after it hadn't been handed in. DS would flat out lie to me about homework. I felt it would be counter productive to have his teachers email me a list of assignments daily (they were willing to do this). I thought waiting for him to come around would give him a bigger sense of achievement, if it ever happened. Also, I didn't want him fighting me openly and refusing to do the homework I had found out about. You can take a horse to water, etc.
I also had a nagging sense that DS might have had problems processing written English. Not enough nagging for me to have him tested for any special ed measures, but some little problem where his brain wasn't processing written language easily. He could recite chunks of Simpsons and South Park dialogue but English Lit bored and frustrated him. He had placed in honours classes in maths and science in his high school placement exam (an aptitude exam, not subject material based) but was mediocre in English language aptitude. I started advising him to look into aviation as a career since he was already interested in planes, flying, WW2 technology, etc., and he seemed to be able to manage adrenaline well. He applied for two universities where he could get a degree and qualify as a pilot. He was accepted in both and started out with the intention of doing psychology and getting his licence. However, he had severe seasonal allergies and another health issue and that precluded passing the FAA medical necessary for flying for a living.
The first year in university was the best thing and the worst thing that ever happened to him. It was a mediocre US university full of people with GPAs like his - the only attraction was the great aviation programme (and a lovely rural campus complete with a huge lake where many students went fishing for their breakfast every morning). His roommate was a pothead who rapped in his sleep (and is now serving a custodial sentence for a violent crime) and all DS did was go to class, go to the library, do a little fishing, eat, and sleep when the roommate wasn't holding parties in the room. The lecture and labwork format of a lot of his subjects was ideal for him and he found he remembered so much from his honours science and maths courses in high school that he quickly got on top of things. He only got one B that entire year - his academic advisor sat him down and asked him if he had ever considered a career in medicine. I want to snog the woman.
After finishing the year he transferred to an excellent university, where he has gone from strength to strength. He has been able to do required arts courses along with his degree courses in science and maths (his degree will be in biology with a minor in chemistry) by carefully choosing options like History of Film, Depictions of African Americans in Film, etc, types of courses to fulfill those requirements - so he is basically looking at a screen for coursework and homework, and not wading through piles of written material for essays which he would have to do in some course like The Roman Republic. He has also been able to download apps that allow him to tape chemistry and other science and maths lectures and some of his courses are available online.
So I guess my advice after all that is:
If your DS is not causing any problems for himself or others outside of your home (drugs, hooliganism, etc) and if he has friends and some interest or hobby, don't worry, and step back from it personally as Maryz and others have said, but try to get him doing something achievable and concrete daily.
Have him do some daily chores around the house, ideally with you or your DH, where you get along amiably, and talk together in a friendly way about stuff other than school. I hope I always gave DS the impression I liked him, and was interested in things he was interested in (tanks of WW2 and Lord of the Rings - he wouldn't read it but the films are a different matter). I always thanked him for help around the house and garden and fair play to him, he was always willing to help.
If you have any suspicion of some special ed need, get it diagnosed. I really regret not doing this with DS, but otoh, I think he would have felt bad about being singled out for any special ed measures. I know that is not how it works but that is how he would have felt about it.