You should try to make him realise the importance of good future in an indirect way,as there is a chance for him to get tempered if directly said,  you should try to realise his reason for being so,  if you are good friends,then it would be better if you advise him,you should make him aware that ,it may be your best years of your life but, still, there is a lot in life for you to face and it would be very difficult to live happily (like you are now) without getting a good job,  you should make him know the tight competition outside.  by doing these many things he may try hard again like he was and may improve in maths as it is a bit easier for him
If he is in 11
tell him/her to solve just 5-7 problems a day
& that can improve him/her slowly and will get interested!!!

No this will not help him he will not do the sums!
animation lecs on youtube will help him
You should be out from